brief biography of rene descartes

René Descartes

Philosopher and mathematician René Descartes is regarded as the father of modern philosophy for defining a starting point for existence, “I think; therefore I am.”

(1596-1650)

Who Was René Descartes?

René Descartes was extensively educated, first at a Jesuit college at age 8, then earning a law degree at 22, but an influential teacher set him on a course to apply mathematics and logic to understanding the natural world. This approach incorporated the contemplation of the nature of existence and of knowledge itself, hence his most famous observation, “I think; therefore I am.”

Descartes was born on March 31, 1596, in La Haye en Touraine, a small town in central France, which has since been renamed after him to honor its most famous son. He was the youngest of three children, and his mother, Jeanne Brochard, died within his first year of life. His father, Joachim, a council member in the provincial parliament, sent the children to live with their maternal grandmother, where they remained even after he remarried a few years later. But he was very concerned with good education and sent René, at age 8, to boarding school at the Jesuit college of Henri IV in La Flèche, several miles to the north, for seven years.

Descartes was a good student, although it is thought that he might have been sickly, since he didn’t have to abide by the school’s rigorous schedule and was instead allowed to rest in bed until midmorning. The subjects he studied, such as rhetoric and logic and the “mathematical arts,” which included music and astronomy, as well as metaphysics, natural philosophy and ethics, equipped him well for his future as a philosopher. So did spending the next four years earning a baccalaureate in law at the University of Poitiers. Some scholars speculate that he may have had a nervous breakdown during this time.

Descartes later added theology and medicine to his studies. But he eschewed all this, “resolving to seek no knowledge other than that of which could be found in myself or else in the great book of the world,” he wrote much later in Discourse on the Method of Rightly Conducting the Reason and Seeking Truth in the Sciences , published in 1637.

So he traveled, joined the army for a brief time, saw some battles and was introduced to Dutch scientist and philosopher Isaac Beeckman, who would become for Descartes a very influential teacher. A year after graduating from Poitiers, Descartes credited a series of three very powerful dreams or visions with determining the course of his study for the rest of his life.

Becoming the Father of Modern Philosophy

Descartes is considered by many to be the father of modern philosophy, because his ideas departed widely from current understanding in the early 17th century, which was more feeling-based. While elements of his philosophy weren’t completely new, his approach to them was. Descartes believed in basically clearing everything off the table, all preconceived and inherited notions, and starting fresh, putting back one by one the things that were certain, which for him began with the statement “I exist.” From this sprang his most famous quote: “I think; therefore I am.”

Since Descartes believed that all truths were ultimately linked, he sought to uncover the meaning of the natural world with a rational approach, through science and mathematics—in some ways an extension of the approach Sir Francis Bacon had asserted in England a few decades prior. In addition to Discourse on the Method , Descartes also published Meditations on First Philosophy and Principles of Philosophy , among other treatises.

Although philosophy is largely where the 20th century deposited Descartes—each century has focused on different aspects of his work—his investigations in theoretical physics led many scholars to consider him a mathematician first. He introduced Cartesian geometry, which incorporates algebra; through his laws of refraction, he developed an empirical understanding of rainbows; and he proposed a naturalistic account of the formation of the solar system, although he felt he had to suppress much of that due to Galileo’s fate at the hands of the Inquisition. His concern wasn’t misplaced—Pope Alexander VII later added Descartes’ works to the Index of Prohibited Books.

Later Life, Death and Legacy

Descartes never married, but he did have a daughter, Francine, born in the Netherlands in 1635. He had moved to that country in 1628 because life in France was too bustling for him to concentrate on his work, and Francine’s mother was a maid in the home where he was staying. He had planned to have the little girl educated in France, having arranged for her to live with relatives, but she died of a fever at age 5.

Descartes lived in the Netherlands for more than 20 years but died in Stockholm, Sweden, on February 11, 1650. He had moved there less than a year before, at the request of Queen Christina, to be her philosophy tutor. The fragile health indicated in his early life persisted. He habitually spent mornings in bed, where he continued to honor his dream life, incorporating it into his waking methodologies in conscious meditation, but the queen’s insistence on 5 am lessons led to a bout of pneumonia from which he could not recover. He was 53.

Sweden was a Protestant country, so Descartes, a Catholic, was buried in a graveyard primarily for unbaptized babies. Later, his remains were taken to the abbey of Saint-Germain-des-Prés, the oldest church in Paris. They were moved during the French Revolution, and were put back later—although urban legend has it that only his heart is there and the rest is buried in the Panthéon.

Descartes’ approach of combining mathematics and logic with philosophy to explain the physical world turned metaphysical when confronted with questions of theology; it led him to a contemplation of the nature of existence and the mind-body duality, identifying the point of contact for the body with the soul at the pineal gland. It also led him to define the idea of dualism: matter meeting non-matter. Because his previous philosophical system had given man the tools to define knowledge of what is true, this concept led to controversy. Fortunately, Descartes himself had also invented methodological skepticism, or Cartesian doubt, thus making philosophers of us all.

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• Name: René Descartes
• Birth Year: 1596
• Birth date: March 31, 1596
• Birth City: La Haye, Touraine
• Birth Country: France
• Gender: Male
• Best Known For: Philosopher and mathematician René Descartes is regarded as the father of modern philosophy for defining a starting point for existence, “I think; therefore I am.”
• Education and Academia
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• Astrological Sign: Aries
• Jesuit College of Henri IV
• University of Poitiers
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• Death Year: 1650
• Death date: February 11, 1650
• Death City: Stockholm
• Death Country: Sweden
• I think; therefore I am." ("Cogito ergo sum.")
• It is not enough to have a good mind; the main thing is to use it well.
• If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.

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Rene Descartes biography

Early Life Rene Descartes

Rene Descartes was born in La Haye en Touraine, France on 31 March 1596. His family were Roman Catholics, though they lived in a Protestant Huguenots area of Poitou. His mother died when he was one year old, and he was brought up by his grandmother and great uncle.

The young Descartes studied at a Jesuit College in La Flèche, where he received a modern education, including maths, physics and the recent works of Galileo. After college, he studied at the University of Poitiers to gain a degree in law.

In 1616, he travelled to Paris in order to practise as a lawyer – according to the wishes of his father. But, Descartes was restless in practising law, he travelled frequently seeking to gain a variety of experiences. In 1618, he joined the Dutch States Army in Breda, where he concentrated on the study of military engineering, which included more study of mathematics.

Visions of a new philosophy

In November 1619, while Descartes was stationed in Neuburg an der Donau, he stated that he received heavenly visions, while he was shut in his room. He felt a divine spirit had infused his mind with the vision of a new philosophy and also the idea of combining mathematics and philosophy.

Descartes had always sought to be independently minded – never relying on books he read; this vision increased his independence of thought and is a characteristic aspect of his philosophy and mathematical work.

In 1620, Descartes left the army and visited several countries before returning to France. He was now motivated to write his own philosophical treatises. His first work was Regulae ad directionem ingenii (1928) Rules for the Direction of the Mind . It set out some of Descartes principles for philosophy and the sciences. In particular, it expressed the importance of relying on reason and the use of mental faculties to methodically work out the truth.

Rule III states:

“As regards any subject we propose to investigate, we must inquire not what other people have thought, or what we ourselves conjecture, but what we can clearly and manifestly perceive by intuition or deduce with certainty.”

– Rene Descartes

Descartes frequently moved in his early years, but he came to settle in the Netherlands, and it was here that he did most of his writings. As well as philosophy, Descartes continued his mathematical studies. He enrolled in Leiden University and studied mathematics and astronomy.

Discourse on the Method

In 1637, Descartes published some of his most important works, including Discours de la méthode . This stated, with Descartes’ characteristic clarity, the importance of methodically never accepting anything as true – which had not been properly examined.

Although Descartes remained a committed Catholic throughout his life, his writings were still controversial for the time period. In 1633, Galileo’s works were put on the prohibited list, and his own Cartesian philosophy was condemned at the University of Utrecht. In 1663, shortly after his death, his works were placed on the Index of Prohibited Works by the Pope.

Ironically, Descartes claimed his meditations were aimed to defend the Catholic faith – through the use of reason and not just relying on faith. However in retrospective, many believe Descartes’ willingness to start with doubt, marked an important shift in philosophy and religious faith. No longer was Descartes stating that the authority of the church and scripture should be assumed – Descartes shifted the proof of truth onto human reason; this was a very influential aspect of the Enlightenment and marked an erosion of authority by the Church.

Descartes willingness to doubt the existence of God led many contemporaries to doubt his real faith. Some believed Descartes might have been a secret Deist, who gave the impression of being a committed Catholic for practical reasons. A biographer of Descartes, Gaukroger states that Descartes remained a committed Catholic throughout his life – but, he had a complementary desire to discover the truth through reason.

Moral Philosophy

Queen Christina of Sweden and Rene Descartes

Descartes wrote on a variety of subjects, relating to philosophy. In 1649, he published 1649 Les Passions de l’âme (Passions of the Soul) a work which sprung from a long correspondence with Princess Elisabeth of Bohemia concerning issues of morality and psychology. This work led to Descartes being invited to the Royal Court of Sweden by Queen Christina.

In 1650, Descartes reluctantly travelled to Sweden and gave the Queen several philosophy lessons early in the morning. However, it was not a success; there was a lack of understanding between the two. More seriously, in the cold castle, Descartes contracted a form of pneumonia, and he died shortly later on 11 February 1650.

Personal life

He had a relationship with a servant girl Helena Jans van der Strom. With Helena, he fathered a child and was distraught when she died in 1640.

Descartes pioneered a new approach to modern philosophy, which was different from the preceding Aristotelian approach. Descartes took great pride in stating that his conclusions were reached from his own deduction and not relying on the works of others.

Descartes methodology of philosophy involved beginning with a metaphysical doubt about everything and from this basis of ‘being unsure of anything’ – seeing what he could prove to be true.

Cogito ergo Sum (“I think therefore I am”)

From this basis of doubt, Descartes started by deducting that the first thing he could be sure of was his own thoughts. If he was doubting, then there must be someone doing the doubting. It is this that led to his famous dictum Cogito Ergo Sum (“I think therefore I am”). Descartes believed that it was only his capacity to think and deduction that was reliable – he believed relying on the senses were open to doubt.

From this premise, Descartes was able to offer an ontological proof of a benevolent God  Descartes believed that the idea of God’s existence was immediately inferable from a “clear and distinct” idea of a supremely perfect being.

An aspect of Descartes philosophy was the distinction of the mind (or soul) and body. Descartes wrote how the mind could control the body and vice versa.

From Descartes’ philosophy, he later developed a form of moral philosophy which were effective for functioning in the real world. This involved obeying laws and customs of the country in which he lived, avoiding extremes and adopting practices which were judicious to those around you. His third principle expresses the importance he attached to controlling the mind and desires.

“Endeavor always to conquer myself rather than fortune, and change my desires rather than the order of the world, and in general, accustom myself to the persuasion that, except our own thoughts, there is nothing absolutely in our power; so that when we have done our best in things external to us, our ill-success cannot possibly be failure on our part.”

Rene Descartes, Discourse on the Method, Chapter III

Mathematics

Descartes developed Cartesian or analytic geometry, which uses algebra to describe geometry. His work on algebra was influential in the later work of Isaac Newton (on calculus and cubic equations) and Gottfried Leibniz (infinitesimal calculus).

Descartes was also a polymath, who also investigated a wide range of fields, including optics, astronomy and music.

Major works of Descartes include

1618. Musicae Compendium. – a work on the aesthetics of music

1626–1628. Regulae ad directionem ingenii (Rules for the Direction of the Mind).

1637. Discours de la méthode (Discourse on the Method).

1637. La Géométrie (Geometry).

1641. Meditationes de Prima Philosophia (Meditations on First Philosophy),

1644. Principia Philosophiae (Principles of Philosophy)

1649. Les passions de l’âme (Passions of the Soul). Dedicated to Princess Elisabeth of the Palatinate.

Citation: Pettinger, Tejvan . “Biography of Rene Descartes”, Oxford, www.biographyonline.net 31 March 2015.

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Internet Encyclopedia of Philosophy

René descartes (1596—1650).

Descartes attempted to address the former issue via his method of doubt. His basic strategy was to consider false any belief that falls prey to even the slightest doubt. This “hyperbolic doubt” then serves to clear the way for what Descartes considers to be an unprejudiced search for the truth . This clearing of his previously held beliefs then puts him at an epistemological ground-zero. From here Descartes sets out to find something that lies beyond all doubt. He eventually discovers that “I exist” is impossible to doubt and is, therefore, absolutely certain. It is from this point that Descartes proceeds to demonstrate God ’s existence and that God cannot be a deceiver. This, in turn, serves to fix the certainty of everything that is clearly and distinctly understood and provides the epistemological foundation Descartes set out to find.

Once this conclusion is reached, Descartes can proceed to rebuild his system of previously dubious beliefs on this absolutely certain foundation. These beliefs, which are re-established with absolute certainty, include the existence of a world of bodies external to the mind, the dualistic distinction of the immaterial mind from the body, and his mechanistic model of physics based on the clear and distinct ideas of geometry. This points toward his second, major break with the Scholastic Aristotelian tradition in that Descartes intended to replace their system based on final causal explanations with his system based on mechanistic principles. Descartes also applied this mechanistic framework to the operation of plant, animal and human bodies, sensation and the passions. All of this eventually culminating in a moral system based on the notion of “generosity.”

The presentation below provides an overview of Descartes’ philosophical thought as it relates to these various metaphysical, epistemological, religious, moral and scientific issues, covering the wide range of his published works and correspondence.

Table of Contents

• Against Scholasticism
• Descartes’ Project
• Cogito, ergo sum
• The Nature of the Mind and its Ideas
• The Causal Arguments
• The Ontological Argument
• Absolute Certainty and the Cartesian Circle
• How to Avoid Error
• The Real Distinction
• The Mind-Body Problem
• Existence of the External World
• The Nature of Body
• Animal and Human Bodies
• Sensations and Passions
• The Provisional Moral Code
• Primary Sources
• Secondary Sources

René Descartes was born to Joachim Descartes and Jeanne Brochard on March 31, 1596 in La Haye, France near Tours. He was the youngest of the couple’s three surviving children. The oldest child, Pierre, died soon after his birth on October 19, 1589. His sister, Jeanne, was probably born sometime the following year, while his surviving older brother, also named Pierre, was born on October 19, 1591. The Descartes clan was a bourgeois family composed of mostly doctors and some lawyers. Joachim Descartes fell into this latter category and spent most of his career as a member of the provincial parliament.

After the death of their mother, which occurred soon after René’s birth, the three Descartes children were sent to their maternal grandmother, Jeanne Sain, to be raised in La Haye and remained there even after their father remarried in 1600. Not much is known about his early childhood, but René is thought to have been a sickly and fragile child, so much so that when he was sent to board at the Jesuit college at La Fleche on Easter of 1607. There, René was not obligated to rise at 5:00am with the other boys for morning prayers but was allowed to rest until 10:00am mass. At La Fleche, Descartes completed the usual courses of study in grammar and rhetoric and the philosophical curriculum with courses in the “verbal arts” of grammar, rhetoric and dialectic (or logic) and the “mathematical arts” comprised of arithmetic, music, geometry and astronomy. The course of study was capped off with courses in metaphysics, natural philosophy and ethics. Descartes is known to have disdained the impractical subjects despite having an affinity for the mathematical curriculum. But, all things considered, he did receive a very broad liberal arts education before leaving La Fleche in 1614.

Little is known of Descartes’ life from 1614-1618. But what is known is that during 1615-1616 he received a degree and a license in civil and canon law at the University of Poiters. However, some speculate that from 1614-1615 Descartes suffered a nervous breakdown in a house outside of Paris and that he lived in Paris from 1616-1618. The story picks up in the summer of 1618 when Descartes went to the Netherlands to become a volunteer for the army of Maurice of Nassau. It was during this time that he met Isaac Beekman, who was, perhaps, the most important influence on his early adulthood. It was Beekman who rekindled Descartes’ interest in science and opened his eyes to the possibility of applying mathematical techniques to other fields. As a New Year’s gift to Beekman, Descartes composed a treatise on music, which was then considered a branch of mathematics, entitled  Compendium Musicae . In 1619 Descartes began serious work on mathematical and mechanical problems under Beekman’s guidance and, finally, left the service of Maurice of Nassau, planning to travel through Germany to join the army of Maximilian of Bavaria.

It is during this year (1619) that Descartes was stationed at Ulm and had three dreams that inspired him to seek a new  method for scientific inquiry and to envisage a unified science. Soon afterwards, in 1620, he began looking for this new method, starting but never completing several works on method, including drafts of the first eleven rules of  Rules for the Direction of the Mind . Descartes worked on and off on it for years until it was finally abandoned for good in 1628. During this time, he also worked on other, more scientifically oriented projects such as optics. In the course of these inquiries, it is possible that he discovered the law of refraction as early as 1626. It is also during this time that Descartes had regular contact with Father Marin Mersenne, who was to become his long time friend and contact with the intellectual community during his 20 years in the Netherlands.

Descartes moved to the Netherlands in late 1628 and, despite several changes of address and a few trips back to France, he remained there until moving to Sweden at the invitation of Queen Christina in late 1649. He moved to the Netherlands in order to achieve solitude and quiet that he could not attain with all the distractions of Paris and the constant intrusion of visitors. It is here in 1629 that Descartes began work on “a little treatise,” which took him approximately three years to complete, entitled  The World . This work was intended to show how mechanistic physics could explain the vast array of phenomena in the world without reference to the Scholastic principles of substantial forms and real qualities, while also asserting a heliocentric conception of the solar system. But the condemnation of Galileo by the Inquisition for maintaining this latter thesis led Descartes to suppress its publication. From 1634-1636, Descartes finished his scientific essays  Dioptique and  Meteors , which apply his geometrical method to these fields. He also wrote a preface to these essays in the winter of 1635/1636 to be attached to them in addition to another one on geometry. This “preface” became  The Discourse on Method and was published in French along with the three essays in June 1637. And, on a personal note, during this time his daughter, Francine, was born in 1635, her mother being a maid at the home where Descartes was staying. But Francine, at the age of five, died of a fever in 1640 when he was making arrangements for her to live with relatives in France so as to ensure her education.

Descartes began work on  Meditations on First Philosophy in 1639. Through Mersenne, Descartes solicited criticism of his  Meditations from amongst the most learned people of his day, including Antoine Arnauld, Peirre Gassendi, and Thomas Hobbes . The first edition of the  Meditations was published in Latin in 1641 with six sets of objections and his replies. A second edition published in 1642 also included a seventh set of objections and replies as well as a letter to Father Dinet in which Descartes defended his system against charges of unorthodoxy. These charges were raised at the Universities of Utrecht and Leiden and stemmed from various misunderstandings about his method and the supposed opposition of his theses to Aristotle and the Christian faith.

This controversy led Descartes to post two open letters against his enemies. The first is entitled  Notes on a Program posted in 1642 in which Descartes refutes the theses of his recently estranged disciple, Henricus Regius, a professor of medicine at Utrecht. These  Notes were intended not only to refute what Descartes understood to be Regius’ false theses but also to distance himself from his former disciple, who had started a ruckus at Utrecht by making unorthodox claims about the nature of human beings. The second is a long attack directed at the rector of Utrecht, Gisbertus Voetius in the  Open Letter to Voetius posted in 1643. This was in response to a pamphlet anonymously circulated by some of Voetius’ friends at the University of Leiden further attacking Descartes’ philosophy. Descartes’  Open Letter led Voetius to have him summoned before the council of Utrecht, who threatened him with expulsion and the public burning of his books. Descartes, however, was able to flee to the Hague and convince the Prince of Orange to intervene on his behalf.

In the following year (1643), Descartes began an affectionate and philosophically fruitful correspondence with Princess Elizabeth of Bohemia, who was known for her acute intellect and had read the  Discourse on Method . Yet, as this correspondence with Elizabeth was beginning, Descartes was already in the midst of writing a textbook version of his philosophy entitled  Principles of Philosophy , which he ultimately dedicated to her. Although it was originally supposed to have six parts, he published it in 1644 with only four completed: The Principles of Human Knowledge, The Principles of Material Things, The Visible Universe, and The Earth. The other two parts were to be on plant and animal life and on human beings, but he decided it would be impossible for him to conduct all the experiments necessary for writing them. Elizabeth probed Descartes about issues that he had not dealt with in much detail before, including free will, the passions and morals. This eventually inspired Descartes to write a treatise entitled  The Passions of the Soul , which was published just before his departure to Sweden in 1649. Also, during these later years, the  Meditations and  Principles were translated from Latin into French for a wider, more popular audience and were published in 1647.

In late 1646, Queen Christina of Sweden initiated a correspondence with Descartes through a French diplomat and friend of Descartes’ named Chanut. Christina pressed Descartes on moral issues and a discussion of the absolute good. This correspondence eventually led to an invitation for Descartes to join the Queen’s court in Stockholm in February 1649. Although he had his reservations about going, Descartes finally accepted Christina’s invitation in July of that year. He arrived in Sweden in September 1649 where he was asked to rise at 5:00am to meet the Queen to discuss philosophy, contrary to his usual habit, developed at La Fleche, of sleeping in late,. His decision to go to Sweden, however, was ill-fated, for Descartes caught pneumonia and died on February 11, 1650.

2. The Modern Turn

A. against scholasticism.

Descartes is often called the “Father of Modern Philosophy,” implying that he provided the seed for a new philosophy that broke away from the old in important ways. This “old” philosophy is Aristotle’s as it was appropriated and interpreted throughout the later medieval period. In fact, Aristotelianism was so entrenched in the intellectual institutions of Descartes’ time that commentators argued that evidence for its the truth could be found in the Bible. Accordingly, if someone were to try to refute some main Aristotelian tenet, then he could be accused of holding a position contrary to the word of God and be punished. However, by Descartes’ time, many had come out in some way against one Scholastic-Aristotelian thesis or other. So, when Descartes argued for the implementation of his modern system of philosophy, breaks with the Scholastic tradition were not unprecedented.

Descartes broke with this tradition in at least two fundamental ways. The first was his rejection of substantial forms as explanatory principles in physics. A substantial form was thought to be an immaterial principle of material organization that resulted in a particular thing of a certain kind. The main principle of substantial forms was the final cause or purpose of being that kind of thing. For example, the bird called the swallow. The substantial form of “swallowness” unites with matter so as to organize it for the sake of being a swallow kind of thing. This also means that any dispositions or faculties the swallow has by virtue of being that kind of thing is ultimately explained by the goal or final cause of being a swallow. So, for instance, the goal of being a swallow is the cause of the swallow’s ability to fly. Hence, on this account, a swallow flies for the sake of being a swallow. Although this might be true, it does not say anything new or useful about swallows, and so it seemed to Descartes that Scholastic philosophy and science was incapable of discovering any new or useful knowledge.

Descartes rejected the use of substantial forms and their concomitant final causes in physics precisely for this reason. Indeed, his essay  Meteorology , that appeared alongside the  Discourse on Method , was intended to show that clearer and more fruitful explanations can be obtained without reference to substantial forms but only by way of deductions from the configuration and motion of parts. Hence, his point was to show that mechanistic principles are better suited for making progress in the physical sciences. Another reason Descartes rejected substantial forms and final causes in physics was his belief that these notions were the result of the confusion of the idea of the body with that of the mind. In the Sixth Replies , Descartes uses the Scholastic conception of gravity in a stone, to make his point. On this account, a characteristic goal of being a stone was a tendency to move toward the center of the earth. This explanation implies that the stone has knowledge of this goal, of the center of the earth and of how to get there. But how can a stone know anything, since it does not think? So, it is a mistake to ascribe mental properties like knowledge to entirely physical things. This mistake should be avoided by clearly distinguishing the idea of the mind from the idea of the body. Descartes considered himself to be the first to do this. His expulsion of the metaphysical principles of substantial forms and final causes helped clear the way for Descartes’ new metaphysical principles on which his modern, mechanistic physics was based.

The second fundamental point of difference Descartes had with the Scholastics was his denial of the thesis that all knowledge must come from sensation. The Scholastics were devoted to the Aristotelian tenet that everyone is born with a clean slate, and that all material for intellectual understanding must be provided through sensation. Descartes, however, argued that since the senses sometimes deceive, they cannot be a reliable source for knowledge. Furthermore, the truth of propositions based on sensation is naturally probabilistic and the propositions, therefore, are doubtful premises when used in arguments. Descartes was deeply dissatisfied with such uncertain knowledge. He then replaced the uncertain premises derived from sensation with the absolute certainty of the clear and distinct ideas perceived by the mind alone, as will be explained below.

b. Descartes’ Project

In the preface to the French edition of the  Principles of Philosophy , Descartes uses a tree as a metaphor for his holistic view of philosophy. “The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principal ones, namely medicine, mechanics and morals” (AT IXB 14: CSM I 186). Although Descartes does not expand much more on this image, a few other insights into his overall project can be discerned. First, notice that metaphysics constitutes the roots securing the rest of the tree. For it is in Descartes’ metaphysics where an absolutely certain and secure epistemological foundation is discovered. This, in turn, grounds knowledge of the geometrical properties of bodies, which is the basis for his physics. Second, physics constitutes the trunk of the tree, which grows up directly from the roots and provides the basis for the rest of the sciences. Third, the sciences of medicine, mechanics and morals grow out of the trunk of physics, which implies that these other sciences are just applications of his mechanistic science to particular subject areas. Finally, the fruits of the philosophy tree are mainly found on these three branches, which are the sciences most useful and beneficial to humankind. However, an endeavor this grand cannot be conducted haphazardly but should be carried out in an orderly and systematic way. Hence, before even attempting to plant this tree, Descartes must first figure out a method for doing so.

Aristotle and subsequent medieval dialecticians set out a fairly large, though limited, set of acceptable argument forms known as “syllogisms” composed of a general or major premise, a particular or minor premise and a conclusion. Although Descartes recognized that these syllogistic forms  preserve truth from premises to conclusion such that if the premises are true, then the conclusion must be true, he still found them faulty. First, these premises are supposed to be known when, in fact, they are merely believed, since they express only probabilities based on sensation. Accordingly, conclusions derived from merely probable premises can only be probable themselves, and, therefore, these probable syllogisms serve more to increase doubt rather than knowledge Moreover, the employment of this method by those steeped in the Scholastic tradition had led to such subtle conjectures and plausible arguments that counter-arguments were easily constructed, leading to profound confusion. As a result, the Scholastic tradition had become such a confusing web of arguments, counter-arguments and subtle distinctions that the truth often got lost in the cracks. ( Rules for the Direction of the Mind, AT X 364, 405-406 & 430: CSM I 11-12, 36 & 51-52).

Descartes sought to avoid these difficulties through the clarity and absolute certainty of geometrical-style demonstration. In geometry, theorems are deduced from a set of self-evident axioms and universally agreed upon definitions. Accordingly, direct apprehension of clear, simple and indubitable truths (or axioms) by intuition and deductions from those truths can lead to new and indubitable knowledge. Descartes found this promising for several reasons. First, the ideas of geometry are clear and distinct, and therefore they are easily understood unlike the confused and obscure ideas of sensation. Second, the propositions constituting geometrical demonstrations are not probabilistic conjectures but are absolutely certain so as to be immune from doubt. This has the additional advantage that any proposition derived from some one or combination of these absolutely certain truths will itself be absolutely certain. Hence, geometry’s rules of inference preserve absolutely certain truth from simple, indubitable and intuitively grasped axioms to their deductive consequences unlike the probable syllogisms of the Scholastics.

The choice of geometrical method was obvious for Descartes given his previous success in applying this method to other disciplines like optics. Yet his application of this method to philosophy was not unproblematic due to a revival of  ancient arguments for global or radical skepticism based on the doubtfulness of human reasoning. But Descartes wanted to show that truths both intuitively grasped and deduced are beyond this possibility of doubt. His tactic was to show that, despite the best skeptical arguments, there is at least one intuitive truth that is beyond all doubt and from which the rest of human knowledge can be deduced. This is precisely the project of Descartes’ seminal work,  Meditations on First Philosophy .

In the  First Meditation , Descartes lays out several arguments for doubting all of his previously held beliefs. He first observes that the senses sometimes deceive, for example, objects at a distance appear to be quite small, and surely it is not prudent to trust someone (or something) that has deceived us even once. However, although this may apply to sensations derived under certain circumstances, doesn’t it seem certain that “I am here, sitting by the fire, wearing a winter dressing gown, holding this piece of paper in my hands, and so on”? (AT VII 18: CSM II 13). Descartes’ point is that even though the senses deceive us some of the time, what basis for doubt exists for the immediate belief that, for example, you are reading this article? But maybe the belief of reading this article or of sitting by the fireplace is not based on true sensations at all but on the false sensations found in dreams. If such sensations are just dreams, then it is not really the case that you are reading this article but in fact you are in bed asleep. Since there is no principled way of distinguishing waking life from dreams, any belief based on sensation has been shown to be doubtful. This includes not only the mundane beliefs about reading articles or sitting by the fire but even the beliefs of experimental science are doubtful, because the observations upon which they are based may not be true but mere dream images. Therefore, all beliefs based on sensation have been called into doubt, because it might all be a dream.

This, however, does not pertain to mathematical beliefs, since they are not based on sensation but on reason. For even though one is dreaming, for example, that, 2 + 3 = 5, the certainty of this proposition is not called into doubt, because 2 + 3 = 5 whether the one believing it is awake or dreaming. Descartes continues to wonder about whether or not God could make him believe there is an earth, sky and other extended things when, in fact, these things do not exist at all. In fact, people sometimes make mistakes about things they think are most certain such as mathematical calculations. But maybe people are not mistaken just some of the time but all of the time such that believing that 2 + 3 = 5 is some kind of persistent and collective mistake, and so the sum of 2 + 3 is really something other than 5. However, such universal deception seems inconsistent with God’s supreme goodness. Indeed, even the occasional deception of mathematical miscalculation also seems inconsistent with God’s goodness, yet people do sometimes make mistakes. Then, in line with the skeptics, Descartes supposes, for the sake of his method, that God does not exist, but instead there is an evil demon with supreme power and cunning that puts all his efforts into deceiving him so that he is always mistaken about everything, including mathematics.

In this way, Descartes called all of his previous beliefs into doubt through some of the best skeptical arguments of his day But he was still not satisfied and decided to go a step further by considering false any belief that falls prey to even the slightest doubt. So, by the end of the  First Meditation , Descartes finds himself in a whirlpool of false beliefs. However, it is important to realize that these doubts and the supposed falsehood of all his beliefs are for the sake of his method: he does not really believe that he is dreaming or is being deceived by an evil demon; he recognizes that his doubt is merely hyperbolic. But the point of this “methodological” or ‘hyperbolic” doubt is to clear the mind of preconceived opinions that might obscure the truth. The goal then is to find something that cannot be doubted even though an evil demon is deceiving him and even though he is dreaming. This first indubitable truth will then serve as an intuitively grasped metaphysical “axiom” from which absolutely certain knowledge can be deduced. For more, see  Cartesian skepticism .

4. The Mind

A. cogito, ergo sum.

In the  Second Meditation , Descartes tries to establish absolute certainty in his famous reasoning:  Cogito, ergo sum or “I think, therefore I am.” These  Meditations are conducted from the first person perspective, from Descartes.’ However, he expects his reader to meditate along with him to see how his conclusions were reached. This is especially important in the  Second Meditation where the intuitively grasped truth of “I exist” occurs. So the discussion here of this truth will take place from the first person or “I” perspective. All sensory beliefs had been found doubtful in the previous meditation, and therefore all such beliefs are now considered false. This includes the belief that I have a body endowed with sense organs. But does the supposed falsehood of this belief mean that I do not exist? No, for if I convinced myself that my beliefs are false, then surely there must be an “I” that was convinced. Moreover, even if I am being deceived by an evil demon, I must exist in order to be deceived at all. So “I must finally conclude that the proposition, ‘I am,’ ‘I exist,’ is necessarily true whenever it is put forward by me or conceived in my mind” (AT VII 25: CSM II 16-17). This just means that the mere fact that I am thinking, regardless of whether or not what I am thinking is true or false, implies that there must be something engaged in that activity, namely an “I.” Hence, “I exist” is an indubitable and, therefore, absolutely certain belief that serves as an axiom from which other, absolutely certain truths can be deduced.

b. The Nature of the Mind and its Ideas

The  Second Meditation continues with Descartes asking, “What am I?” After discarding the traditional Scholastic-Aristotelian concept of a human being as a rational animal due to the inherent difficulties of defining “rational” and “animal,” he finally concludes that he is a thinking thing, a mind: “A thing that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and has sense perceptions” (AT VII 28: CSM II 19). In the  Principles , part I, sections 32 and 48, Descartes distinguishes intellectual perception and volition as what properly belongs to the nature of the mind alone while imagination and sensation are, in some sense, faculties of the mind insofar as it is united with a body. So imagination and sensation are faculties of the mind in a weaker sense than intellect and will, since they require a body in order to perform their functions. Finally, in the  Sixth Meditation , Descartes claims that the mind or “I” is a non-extended thing. Now, since extension is the nature of body, is a necessary feature of body, it follows that the mind is by its nature not a body but an immaterial thing. Therefore, what I am is an immaterial thinking thing with the faculties of intellect and will.

It is also important to notice that the mind is a substance and the modes of a thinking substance are its ideas. For Descartes a substance is a thing requiring nothing else in order to exist. Strictly speaking, this applies only to God whose existence is his essence, but the term “substance” can be applied to creatures in a qualified sense. Minds are substances in that they require nothing except God’s concurrence, in order to exist. But ideas are “modes” or “ways” of thinking, and, therefore, modes are not substances, since they must be the ideas of some mind or other. So, ideas require, in addition to God’s concurrence, some created thinking substance in order to exist (see  Principles of Philosophy , part I, sections 51 & 52). Hence the mind is an immaterial thinking substance, while its ideas are its modes or ways of thinking.

Descartes continues on to distinguish three kinds of ideas at the beginning of the  Third Meditation , namely those that are fabricated, adventitious, or innate. Fabricated ideas are mere inventions of the mind. Accordingly, the mind can control them so that they can be examined and set aside at will and their internal content can be changed. Adventitious ideas are sensations produced by some material thing existing externally to the mind. But, unlike fabrications, adventitious ideas cannot be examined and set aside at will nor can their internal content be manipulated by the mind. For example, no matter how hard one tries, if someone is standing next to a fire, she cannot help but feel the heat as heat. She cannot set aside the sensory idea of heat by merely willing it as we can do with our idea of Santa Claus, for example. She also cannot change its internal content so as to feel something other than heat–say, cold. Finally, innate ideas are placed in the mind by God at creation. These ideas can be examined and set aside at will but their internal content cannot be manipulated. Geometrical ideas are paradigm examples of innate ideas. For example, the idea of a triangle can be examined and set aside at will, but its internal content cannot be manipulated so as to cease being the idea of a three-sided figure. Other examples of innate ideas would be metaphysical principles like “what is done cannot be undone,” the idea of the mind, and the idea of God.

Descartes’ idea of God will be discussed momentarily, but let’s consider his claim that the mind is better known than the body. This is the main point of the wax example found in the  Second Meditation . Here, Descartes pauses from his methodological doubt to examine a particular piece of wax fresh from the honeycomb:

It has not yet quite lost the taste of the honey; it retains some of the scent of flowers from which it was gathered; its color shape and size are plain to see; it is hard, cold and can be handled without difficulty; if you rap it with your knuckle it makes a sound. (AT VII 30: CSM II 20)

The point is that the senses perceive certain qualities of the wax like its hardness, smell, and so forth. But, as it is moved closer to the fire, all of these sensible qualities change. “Look: the residual taste is eliminated, the smell goes away, the color changes, the shape is lost, the size increases, it becomes liquid and hot” (AT VII 30: CSM II 20). However, despite these changes in what the senses perceive of the wax, it is still judged to be the same wax now as before. To warrant this judgment, something that does not change must have been perceived in the wax.

This reasoning establishes at least three important points. First, all sensation involves some sort of judgment, which is a mental mode. Accordingly, every sensation is, in some sense, a mental mode, and “the more attributes [that is, modes] we discover in the same thing or substance, the clearer is our knowledge of that substance” (AT VIIIA 8: CSM I 196). Based on this principle, the mind is better known than the body, because it has ideas about both extended and mental things and not just of extended things, and so it has discovered more modes in itself than in bodily substances. Second, this is also supposed to show that what is unchangeable in the wax is its extension in length, breadth and depth, which is not perceivable by the senses but by the mind alone. The shape and size of the wax are modes of this extension and can, therefore, change. But the extension constituting this wax remains the same and permits the judgment that the body with the modes existing in it after being moved by the fire is the same body as before even though all of its sensible qualities have changed. One final lesson is that Descartes is attempting to wean his reader from reliance on sense images as a source for, or an aid to, knowledge. Instead, people should become accustomed to thinking without images in order to clearly understand things not readily or accurately represented by them, for example, God and the mind. So, according to Descartes, immaterial, mental things are better known and, therefore, are better sources of knowledge than extended things.

a. The Causal Arguments

At the beginning of the  Third Meditation only “I exist” and “I am a thinking thing” are beyond doubt and are, therefore, absolutely certain. From these intuitively grasped, absolutely certain truths, Descartes now goes on to deduce the existence of something other than himself, namely God. Descartes begins by considering what is necessary for something to be the adequate cause of its effect. This will be called the “Causal Adequacy Principle” and is expressed as follows: “there must be at least as much reality in the efficient and total cause as in the effect of that cause,” which in turn implies that something cannot come from nothing (AT VII 40: CSM II 28). Here Descartes is espousing a causal theory that implies whatever is possessed by an effect must have been given to it by its cause. For example, when a pot of water is heated to a boil, it must have received that heat from some cause that had at least that much heat. Moreover, something that is not hot enough cannot cause water to boil, because it does not have the requisite reality to bring about that effect. In other words, something cannot give what it does not have.

Descartes goes on to apply this principle to the cause of his ideas. This version of the Causal Adequacy Principle states that whatever is contained objectively in an idea must be contained either formally or eminently in the cause of that idea. Definitions of some key terms are now in order. First, the objective reality contained in an idea is just its representational content; in other words, it is the “object” of the idea or what that idea is about. The idea of the sun, for instance, contains the reality of the sun in it objectively. Second, the formal reality contained in something is a reality actually contained in that thing. For example, the sun itself has the formal reality of extension since it is actually an extended thing or body. Finally, a reality is contained in something eminently when that reality is contained in it in a higher form such that (1) the thing does not possess that reality formally, but (2) it has the ability to cause that reality formally in something else. For example, God is not formally an extended thing but solely a thinking thing; however, he is eminently the extended universe in that it exists in him in a higher form, and accordingly he has the ability to cause its existence. The main point is that the Causal Adequacy Principle also pertains to the causes of ideas so that, for instance, the idea of the sun must be caused by something that contains the reality of the sun either actually (formally) or in some higher form (eminently).

Once this principle is established, Descartes looks for an idea of which he could not be the cause. Based on this principle, he can be the cause of the objective reality of any idea that he has either formally or eminently. He is formally a finite substance, and so he can be the cause of any idea with the objective reality of a finite substance. Moreover, since finite substances require only God’s concurrence to exist and modes require a finite substance and God, finite substances are more real than modes. Accordingly, a finite substance is not formally but eminently a mode, and so he can be the cause of all his ideas of modes. But the idea of God is the idea of an infinite substance. Since a finite substance is less real than an infinite substance by virtue of the latter’s absolute independence, it follows that Descartes, a finite substance, cannot be the cause of his idea of an infinite substance. This is because a finite substance does not have enough reality to be the cause of this idea, for if a finite substance were the cause of this idea, then where would it have gotten the extra reality? But the idea must have come from something. So something that is actually an infinite substance, namely God, must be the cause of the idea of an infinite substance. Therefore, God exists as the only possible cause of this idea.

Notice that in this argument Descartes makes a direct inference from having the idea of an infinite substance to the actual existence of God. He provides another argument that is cosmological in nature in response to a possible objection to this first argument. This objection is that the cause of a finite substance with the idea of God could also be a finite substance with the idea of God. Yet what was the cause of that finite substance with the idea of God? Well, another finite substance with the idea of God. But what was the cause of that finite substance with the idea of God? Well, another finite substance . . . and so on to infinity. Eventually an ultimate cause of the idea of God must be reached in order to provide an adequate explanation of its existence in the first place and thereby stop the infinite regress. That ultimate cause must be God, because only he has enough reality to cause it. So, in the end, Descartes claims to have deduced God’s existence from the intuitions of his own existence as a finite substance with the idea of God and the Causal Adequacy Principle, which is “manifest by the natural light,” thereby indicating that it is supposed to be an absolutely certain intuition as well.

b. The Ontological Argument

The  ontological argument is found in the  Fifth Meditation and follows a more straightforwardly geometrical line of reasoning. Here Descartes argues that God’s existence is deducible from the idea of his nature just as the fact that the sum of the interior angles of a triangle are equal to two right angles is deducible from the idea of the nature of a triangle. The point is that this property is contained in the nature of a triangle, and so it is inseparable from that nature. Accordingly, the nature of a triangle without this property is unintelligible. Similarly, it is apparent that the idea of God is that of a supremely perfect being, that is, a being with all perfections to the highest degree. Moreover, actual existence is a perfection, at least insofar as most would agree that it is better to actually exist than not. Now, if the idea of God did not contain actual existence, then it would lack a perfection. Accordingly, it would no longer be the idea of a supremely perfect being but the idea of something with an imperfection, namely non-existence, and, therefore, it would no longer be the idea of God. Hence, the idea of a supremely perfect being or God without existence is unintelligible. This means that existence is contained in the essence of an infinite substance, and therefore God must exist by his very nature. Indeed, any attempt to conceive of God as not existing would be like trying to conceive of a mountain without a valley – it just cannot be done.

6. The Epistemological Foundation

A. absolute certainty and the cartesian circle.

Recall that in the  First Meditation Descartes supposed that an evil demon was deceiving him. So as long as this supposition remains in place, there is no hope of gaining any absolutely certain knowledge. But he was able to demonstrate God’s existence from intuitively grasped premises, thereby providing, a glimmer of hope of extricating himself from the evil demon scenario. The next step is to demonstrate that God cannot be a deceiver. At the beginning of the  Fourth Meditation, Descartes claims that the will to deceive is “undoubtedly evidence of malice or weakness” so as to be an imperfection. But, since God has all perfections and no imperfections, it follows that God cannot be a deceiver. For to conceive of God with the will to deceive would be to conceive him to be both having no imperfections and having one imperfection, which is impossible; it would be like trying to conceive of a mountain without a valley. This conclusion, in addition to God’s existence, provides the absolutely certain foundation Descartes was seeking from the outset of the  Meditations . It is absolutely certain because both conclusions (namely that God exists and that God cannot be a deceiver) have themselves been demonstrated from immediately grasped and absolutely certain intuitive truths.

This means that God cannot be the cause of human error, since he did not create humans with a faculty for generating them, nor could God create some being, like an evil demon, who is bent on deception. Rather, humans are the cause of their own errors when they do not use their faculty of judgment correctly. Second, God’s non-deceiving nature also serves to guarantee the truth of all clear and distinct ideas. So God would be a deceiver, if there were a clear and distinct idea that was false, since the mind cannot help but believe them to be true. Hence, clear and distinct ideas must be true on pain of contradiction. This also implies that knowledge of God’s existence is required for having any absolutely certain knowledge. Accordingly, atheists, who are ignorant of God’s existence, cannot have absolutely certain knowledge of any kind, including scientific knowledge.

But this veridical guarantee gives rise to a serious problem within the  Meditations , stemming from the claim that all clear and distinct ideas are ultimately guaranteed by God’s existence, which is not established until the  Third Meditation . This means that those truths reached in the  Second Meditation , such as “I exist” and “I am a thinking thing,” and those principles used in the  Third Meditation to conclude that God exists, are not clearly and distinctly understood, and so they cannot be absolutely certain. Hence, since the premises of the argument for God’s existence are not absolutely certain, the conclusion that God exists cannot be certain either. This is what is known as the “Cartesian Circle,” because Descartes’ reasoning seems to go in a circle in that he needs God’s existence for the absolute certainty of the earlier truths and yet he needs the absolute certainty of these earlier truths to demonstrate God’s existence with absolute certainty.

Descartes’ response to this concern is found in the  Second Replies . There he argues that God’s veridical guarantee only pertains to the recollection of arguments and not the immediate awaRenéss of an argument’s clarity and distinctness currently under consideration. Hence, those truths reached before the demonstration of God’s existence are clear and distinct when they are being attended to but cannot be relied upon as absolutely certain when those arguments are recalled later on. But once God’s existence has been demonstrated, the recollection of the clear and distinct perception of the premises is sufficient for absolutely certain and, therefore, perfect knowledge of its conclusion (see also the  Fifth Meditation at AT VII 69-70: CSM II XXX).

b. How to Avoid Error

In the  Third Meditation , Descartes argues that only those ideas called “judgments” can, strictly speaking, be true or false, because it is only in making a judgment that the resemblance, conformity or correspondence of the idea to things themselves is affirmed or denied. So if one affirms that an idea corresponds to a thing itself when it really does not, then an error has occurred. This faculty of judging is described in more detail in the  Fourth Meditation . Here judgment is described as a faculty of the mind resulting from the interaction of the faculties of intellect and will. Here Descartes observes that the intellect is finite in that humans do not know everything, and so their understanding of things is limited. But the will or faculty of choice is seemingly infinite in that it can be applied to just about anything whatsoever. The finitude of the intellect along with this seeming infinitude of the will is the source of human error. For errors arise when the will exceeds the understanding such that something laying beyond the limits of the understanding is voluntarily affirmed or denied. To put it more simply: people make mistakes when they choose to pass judgment on things they do not fully understand. So the will should be restrained within the bounds of what the mind understands in order to avoid error. Indeed, Descartes maintains that judgments should only be made about things that are clearly and distinctly understood, since their truth is guaranteed by God’s non-deceiving nature. If one only makes judgments about what is clearly and distinctly understood and abstains from making judgments about things that are not, then error would be avoided altogether. In fact, it would be impossible to go wrong if this rule were unwaveringly followed.

7. Mind-Body Relation

A. the real distinction.

One of Descartes’ main conclusions is that the mind is really distinct from the body. But what is a “real distinction”? Descartes explains it best at  Principles , part 1, section 60. Here he first states that it is a distinction between two or more substances. Second, a real distinction is perceived when one substance can be clearly and distinctly understood without the other and vice versa. Third, this clear and distinct understanding shows that God can bring about anything understood in this way. Hence, in arguing for the real distinction between mind and body, Descartes is arguing that 1) the mind is a substance, 2) it can be clearly and distinctly understood without any other substance, including bodies, and 3) that God could create a mental substance all by itself without any other created substance. So Descartes is ultimately arguing for the possibility of minds or souls existing without bodies.

Descartes argues that mind and body are really distinct in two places in the  Sixth Meditation . The first argument is that he has a clear and distinct understanding of the mind as a thinking, non-extended thing and of the body as an extended, non-thinking thing. So these respective ideas are clearly and distinctly understood to be opposite from one another and, therefore, each can be understood all by itself without the other. Two points should be mentioned here. First, Descartes’ claim that these perceptions are clear and distinct indicates that the mind cannot help but believe them true, and so they must be true for otherwise God would be a deceiver, which is impossible. So the premises of this argument are firmly rooted in his foundation for absolutely certain knowledge. Second, this indicates further that he knows that God can create mind and body in the way that they are being clearly and distinctly understood. Therefore, the mind can exist without the body and vice versa.

The second version is found later in the  Sixth Meditation where Descartes claims to understand the nature of body or extension to be divisible into parts, while the nature of the mind is understood to be “something quite simple and complete” so as not to be composed of parts and is, therefore, indivisible. From this it follows that mind and body cannot have the same nature, for if this were true, then the same thing would be both divisible and not divisible, which is impossible. Hence, mind and body must have two completely different natures in order for each to be able to be understood all by itself without the other. Although Descartes does not make the further inference here to the conclusion that mind and body are two really distinct substances, it nevertheless follows from their respective abilities to be clearly and distinctly understood without each other that God could create one without the other.

b. The Mind-Body Problem

The famous  mind-body problem has its origins in Descartes’ conclusion that mind and body are really distinct. The crux of the difficulty lies in the claim that the respective natures of mind and body are completely different and, in some way, opposite from one another. On this account, the mind is an entirely immaterial thing without any extension in it whatsoever; and, conversely, the body is an entirely material thing without any thinking in it at all. This also means that each substance can have only its kind of modes. For instance, the mind can only have modes of understanding, will and, in some sense, sensation, while the body can only have modes of size, shape, motion, and quantity. But bodies cannot have modes of understanding or willing, since these are not ways of being extended; and minds cannot have modes of shape or motion, since these are not ways of thinking.

The difficulty arises when it is noticed that sometimes the will moves the body, for example, the intention to ask a question in class causes the raising of your arm, and certain motions in the body cause the mind to have sensations. But how can two substances with completely different natures causally interact? Pierre Gassendi in the  Fifth Objections and Princess Elizabeth in her correspondence with Descartes both noted this problem and explained it in terms of contact and motion. The main thrust of their concern is that the mind must be able to come into contact with the body in order to cause it to move. Yet contact must occur between two or more surfaces, and, since having a surface is a mode of extension, minds cannot have surfaces. Therefore, minds cannot come into contact with bodies in order to cause some of their limbs to move. Furthermore, although Gassendi and Elizabeth were concerned with how a mental substance can cause motion in a bodily substance, a similar problem can be found going the other way: how can the motion of particles in the eye, for example, traveling through the optic nerve to the brain cause visual sensations in the mind, if no contact or transfer of motion is possible between the two?

This could be a serious problem for Descartes, because the actual existence of modes of sensation and voluntary bodily movement indicates that mind and body do causally interact. But the completely different natures of mind and body seem to preclude the possibility of this interaction. Hence, if this problem cannot be resolved, then it could be used to imply that mind and body are not completely different but they must have something in common in order to facilitate this interaction. Given Elizabeth’s and Gassendi’s concerns, it would suggest that the mind is an extended thing capable of having a surface and motion. Therefore, Descartes could not really come to a clear and distinct understanding of mind and body independently of one another, because the nature of the mind would have to include extension or body in it.

Descartes, however, never seemed very concerned about this problem. The reason for this lack of concern is his conviction expressed to both Gassendi and Elizabeth that the problem rests upon a misunderstanding about the union between mind and body. Though he does not elaborate to Gassendi, Descartes does provide some insight in a 21 May 1643 letter to Elizabeth. In that letter, Descartes distinguishes between various primitive notions. The first is the notion of the body, which entails the notions of shape and motion. The second is the notion of the mind or soul, which includes the perceptions of the intellect and the inclinations of the will. The third is the notion of the union of the soul with the body, on which depend the notion of the soul’s power to move the body and the body’s power to cause sensations and passions in the soul.

The notions entailed by or included in the primitive notions of body and soul just are the notions of their respective modes. This suggests that the notions depending on the primitive notion of the union of soul and body are the modes of the entity resulting from this union. This would also mean that a human being is one thing instead of two things that causally interact through contact and motion as Elizabeth and Gassendi supposed. Instead, a human being, that is, a soul united with a body, would be a whole that is more than the sum of its parts. Accordingly, the mind or soul is a part with its own capacity for modes of intellect and will; the body is a part with its own capacity for modes of size, shape, motion and quantity; and the union of mind and body or human being, has a capacity for its own set of modes over and above the capacities possessed by the parts alone. On this account, modes of voluntary bodily movement would not be modes of the body alone resulting from its mechanistic causal interaction with a mental substance, but rather they would be modes of the whole human being. The explanation of, for example, raising the arm would be found in a principle of choice internal to human nature and similarly sensations would be modes of the whole human being. Hence, the human being would be causing itself to move and would have sensations and, therefore, the problem of causal interaction between mind and body is avoided altogether. Finally, on the account sketched here, Descartes’ human being is actually one, whole thing, while mind and body are its parts that God  could make exist independently of one another.

However, a final point should be made before closing this section. The position sketched in the previous couple of paragraphs is not the prevalent view among scholars and requires more justification than can be provided here. Most scholars understand Descartes’ doctrine of the real distinction between mind and body in much the same way as Elizabeth and Gassendi did such that Descartes’ human being is believed to be not one, whole thing but two substances that somehow mechanistically interact. This also means that they find the mind-body problem to be a serious, if not fatal, flaw of Descartes’ entire philosophy. But the benefit of the brief account provided here is that it helps explain Descartes’ lack of concern for this issue and his persistent claims that an understanding of the union of mind and body would put to rest people’s concerns about causal interaction via contact and motion.

8. Body and the Physical Sciences

A. existence of the external world.

In the  Sixth Meditation , Descartes recognizes that sensation is a passive faculty that receives sensory ideas from something else. But what is this “something else”? According to the Causal Adequacy Principle of the  Third Meditation , this cause must have at least as much reality either formally or eminently as is contained objectively in the produced sensory idea. It, therefore, must be either Descartes himself, a body or extended thing that actually has what is contained objectively in the sensory idea, or God or some creature more noble than a body, who would possess that reality eminently. It cannot be Descartes, since he has no control over these ideas. It cannot be God or some other creature more noble than a body, for if this were so, then God would be a deceiver, because the very strong inclination to believe that bodies are the cause of sensory ideas would then be wrong; and if it is wrong, there is no faculty that could discover the error. Accordingly, God would be the source of the mistake and not human beings, which means that he would be a deceiver. So bodies must be the cause of the ideas of them, and therefore bodies exist externally to the mind.

b. The Nature of Body

In part II of the  Principles , Descartes argues that the entire physical universe is corporeal substance indefinitely extended in length, breadth, and depth. This means that the extension constituting bodies and the extension constituting the space in which those bodies are said to be located are the same. Here Descartes is rejecting the claim held by some that bodies have something over and above extension as part of their nature, namely impenetrability, while space is just penetrable extension in which impenetrable bodies are located. Therefore, body and space have the same extension in that body is not impenetrable extension and space penetrable extension, but rather there is only one kind of extension. Descartes maintains further that extension entails impenetrability, and hence there is only impenetrable extension. He goes on to state that: “The terms ‘place’ and ‘space,’ then, do not signify anything different from the body which is said to be in a place . . .” (AT VIIIA 47: CSM I 228). Hence, it is not that bodies are in space but that the extended universe is composed of a plurality or plenum of impenetrable bodies. On this account, there is no place in which a particular body is located, but rather what is called a “place” is just a particular body’s relation to other bodies. However, when a body is said to change its place, it merely has changed its relation to these other bodies, but it does not leave an “empty” space behind to be filled by another body. Rather, another body takes the place of the first such that a new part of extension now constitutes that place or space.

Here an example should prove helpful. Consider the example of a full wine bottle. The wine is said to occupy that place within the bottle. Once the wine is finished, this place is now constituted by the quantity of air now occupying it. Notice that the extension of the wine and that of the air are two different sets of bodies, and so the place inside the wine bottle was constituted by two different pieces of extension. But, since these two pieces of extension have the same size, shape and relation to the body surrounding it, that is, the bottle, it is called one and the same “place” even though, strictly speaking, it is made up of two different pieces of extension. Therefore, so long as bodies of the same shape, size and position continue to replace each other, it is considered one and the same place.

This assimilation of a place or space with the body constituting it gives rise to an interesting philosophical problem. Since a place is identical with the body constituting it, how does a place retain its identity and, therefore, remain the “same” place when it is replaced by another body that now constitutes it? A return to the wine bottle example will help to illustrate this point. Recall that first the extension of the wine constituted the place inside the bottle and then, after the wine was finished, that place inside the body was constituted by the extension of the air now occupying it. So, since the wine’s extension is different from the air’s extension, it seems to follow that the place inside the wine bottle is not the exactly same place but two different places at two different times. It is difficult to see how Descartes would address this issue.

Another important consequence of Descartes’ assimilation of bodies and space is that a vacuum or an empty space is unintelligible. This is because an empty space, according to Descartes, would just be a non-extended space, which is impossible. A return to the wine bottle will further illustrate this point. Notice that the place inside the wine bottle was first constituted by the wine and then by air. These are two different kinds of extended things, but they are extended things nonetheless. Accordingly, the place inside the bottle is constituted first by one body (the wine) and then by another (air). But suppose that all extension is removed from the bottle so that there is an “empty space.” Now, distance is a mode requiring extension to exist, for it makes no sense to speak of spatial distance without space or extension. So, under these circumstances, no mode of distance could exist inside the bottle. That is, no distance would exist between the bottle’s sides, and therefore the sides would touch. Therefore, an empty space cannot exist between two or more bodies.

Descartes’ close assimilation of body and space, his rejection of the vacuum, and some textual issues have lead many to infer an asymmetry in his metaphysics of thinking and extended things. This asymmetry is found in the claim that particular minds are substances for Descartes but not particular bodies. Rather, these considerations indicate to some that only the whole, physical universe is a substance, while particular bodies, for example, the wine bottle, are modes of that substance. Though the textual issues are many, the main philosophical problem stems from the rejection of the vacuum. The argument goes like this: particular bodies are not really distinct substances, because two or more particular bodies cannot be clearly and distinctly understood with an empty space between them; that is, they are not separable from each other, even by the power of God. Hence, particular bodies are not substances, and therefore they must be modes. However, this line of reasoning is a result of misunderstanding the criterion for a real distinction. Instead of trying to understand two bodies with an empty space between them, one body should be understood all by itself so that God could have created a world with that body, for example, the wine bottle, as its only existent. Hence, since it requires only God’s concurrence to exist, it is a substance that is really distinct from all other thinking and extended substances. Although difficulties also arise for this argument from Descartes’ account of bodily surfaces as a mode shared between bodies, these are too complex to address here. But, suffice it to say that the textual evidence is also in favor of the claim that Descartes, despite the unforeseen problem about surfaces, maintained that particular bodies are substances. The most telling piece of textual evidence is found in a 1642 letter to Gibeuf:

From the simple fact that I consider two halves of a part of matter, however small it may be, as two complete substances . . . I conclude with certainty that they are really divisible. (AT III 477: CSMK 202-203

These considerations in general, and this quotation in particular, lead to another distinct feature of Cartesian body, namely that extension is infinitely divisible. The point is that no matter how small a piece of matter, it can always be divided in half, and then each half can itself be divided in half, and so on to infinity. These considerations about the vacuum and the infinite divisibility of extension amount to a rejection of atomism. Atomism is a school of thought going back to the ancients, which received a revival in the 17th century most notably in the philosophy and science of Pierre Gassendi. On this account, all change in the universe could be explained by the movements of very small, indivisible particles called “atoms” in a void or empty space. But, if Descartes’ arguments for rejecting the vacuum and the infinite divisibility of matter are sound, then atomism must be false, since the existence of indivisible atoms and an empty space would both be unintelligible.

Descartes devised a non-atomistic, mechanistic physics in which all physical phenomena were to be explain by the configuration and motion of a body’s miniscule parts. This mechanistic physics is also a point of fundamental difference between the Cartesian and Scholastic-Aristotelian schools of thought. For the latter (as Descartes understood them), the regular behavior of inanimate bodies was explained by certain ends towards which those bodies strive. Descartes, on the other hand, thought human effort is better directed toward the discovery of the mechanistic causes of things given the uselessness of final causal explanations and how it is vain to seek God’s purposes. Furthermore, Descartes maintained that the geometric method should also be applied to physics so that results are deduced from the clear and distinct perceptions of the geometrical or quantifiable properties found in bodies, that is, size, shape, motion, determination (or direction), quantity, and so forth.

Perhaps the most concise summary of Descartes’ general view of the physical universe is found in part III, section 46 of the  Principles :

From what has already been said we have established that all the bodies in the universe are composed of one and the same matter, which is divisible into indefinitely many parts, and is in fact divided into a large number of parts which move in different directions and have a sort of circular motion; moreover, the same quantity of motion is always preserved in the universe. (AT VIIIA 100: CSM I 256)

Since the matter constituting the physical universe and its divisibility were previously discussed, a brief explanation of the circular motion of bodies and the preservation of motion is in order. The first thesis is derived from God’s immutability and implies that no quantity of motion is ever added to or subtracted from the universe, but rather quantities of motion are merely passed from one body to another. God’s immutability is also used to support the first law of motion, which is that “each and everything, in so far as it can, always continues in the same state; and thus what is once in motion always continues in motion” (AT VIIIA 62-63: CSM I 241). This principle indicates that something will remain in a given state as long as it is not being affected by some external cause. So a body moving at a certain speed will continue to move at that speed indefinitely unless something comes along to change it. The second thesis about the circular motion of bodies is discussed at  Principles , part II, section 33. This claim is based on the earlier thesis that the physical universe is a plenum of contiguous bodies. On this account, one moving body must collide with and replace another body, which, in turn, is set in motion and collides with another body, replacing it and so on. But, at the end of this series of collisions and replacements, the last body moved must then collide with and replace the first body in the sequence. To illustrate: suppose that body A collides with and replaces body B, B replaces C, C replaces D, and then D replaces A. This is known as a Cartesian vortex.

Descartes’ second law of motion is that “all motion is in itself rectilinear; and hence any body moving in a circle always tends to move away from the center of the circle which it describes” (AT VIIIA 63-64: CSM I 241-242). This is justified by God’s immutability and simplicity in that he will preserve a quantity of motion in the exact form in which it is occurring until some created things comes along to change it. The principle expressed here is that any body considered all by itself tends to move in a straight line unless it collides with another body, which deflects it. Notice that this is a thesis about any body left all by itself, and so only lone bodies will continue to move in a straight line. However, since the physical world is a plenum, bodies are not all by themselves but constantly colliding with one another, which gives rise to Cartesian vortices as explained above.

The third general law of motion, in turn, governs the collision and deflection of bodies in motion. This third law is that “if a body collides with another body that is stronger than itself, it loses none of its motion; but if it collides with a weaker body, it loses a quantity of motion” (AT VIIIA 65: CSM I 242). This law expresses the principle that if a body’s movement in a straight line is less resistant than a stronger body with which it collides, then it won’t lose any of its motion but its direction will be changed. But if the body collides with a weaker body, then the first body loses a quantity of motion equal to that given in the second. Notice that all three of these principles doe not employ the goals or purposes (that is, final causes) utilized in Scholastic-Aristotelian physics as Descartes understood it but only the most general laws of the mechanisms of bodies by means of their contact and motion.

d. Animal and Human Bodies

In part five of the  Discourse on Method , Descartes examines the nature of animals and how they are to be distinguished from human beings. Here Descartes argues that if a machine were made with the outward appearance of some animal lacking reason, like a monkey, it would be indistinguishable from a real specimen of that animal found in nature. But if such a machine of a human being were made, it would be readily distinguishable from a real human being due to its inability to use language. Descartes’ point is that the use of language is a sign of rationality and only things endowed with minds or souls are rational. Hence, it follows that no animal has an immaterial mind or soul. For Descartes this also means that animals do not, strictly speaking, have sensations like hunger, thirst and pain. Rather, squeals of pain, for instance, are mere mechanical reactions to external stimuli without any sensation of pain. In other words, hitting a dog with a stick, for example, is a kind of input and the squeal that follows would be merely output, but the dog did not feel anything at all and could not feel pain unless it was endowed with a mind. Humans, however, are endowed with minds or rational souls, and therefore they can use language and feel sensations like hunger, thirst, and pain. Indeed, this Cartesian “fact” is at the heart of Descartes’ argument for the union of the mind with the body summarized near the end of part five of the  Discourse and laid out in full in the  Sixth Meditation .

Yet Descartes still admits that both animal and human bodies can be best understood to be “machine[s] made of earth, which God forms.” (AT XI 120: CSM I 99). The point is that just as the workings of a clock can be best understood by means of the configuration and motion of its parts so also with animal and human bodies. Indeed, the heart of an animal and that of a human being are so much alike that he advises the reader unversed in anatomy “to have the heart of some large animal with lungs dissected before him (for such a heart is in all respects sufficiently like that of a man), and be shown the two chambers or cavities which are present in it” (AT VI 47: CSM I 134). He then goes on to describe in some detail the motion of the blood through the heart in order to explain that when the heart hardens it is not contracting but really swelling in such a way as to allow more blood into a given cavity. Although this account goes contrary to the (more correct) observation made by William Harvey, an Englishman who published a book on the circulation of the blood in 1628, Descartes argues that his explanation has the force of geometrical demonstration. Accordingly, the physiology and biology of human bodies, considered without regard for those functions requiring the soul to operate, should be conducted in the same way as the physiology and biology of animal bodies, namely via the application of the geometrical method to the configuration and motion of parts.

9. Sensations and Passions

In his last published work,  Passions of the Soul , Descartes provides accounts of how various motions in the body cause sensations and passions to arise in the soul. He begins by making several observations about the mind-body relation. The whole mind is in the whole body and the whole in each of its parts but yet its primary seat is in a little gland at the center of the brain now known as the “pineal gland.” Descartes is not explicit about what he means by “the whole mind in the whole body and the whole in each of its parts.” But this was not an uncommon way of characterizing how the soul is united to the body at Descartes’ time. The main point was that the soul makes a human body truly human; that is, makes it a living human body and not merely a corpse. Given Descartes’ unexplained use of this phrase, it is reasonable to suppose that he used it in the way his contemporaries would have understood it. So the mind is united to the whole body and the whole in each of its parts insofar as it is a soul or principle of life. Accordingly, the body’s union with the soul makes it a living human body or a human body, strictly speaking (see letter to Mersenne dated 9 February 1645). But, the “primary seat”, that is, the place where the soul performs its primary functions, is the point where the mind is, in some sense, affected by the body, namely the pineal gland.

Descartes maintains further that all sensations depend on the nerves, which extend from the brain to the body’s extremities in the form of tiny fibers encased by tube-like membranes. These fibers float in a very fine matter known as the “animal spirits.” This allows these fibers to float freely so that anything causing the slightest motion anywhere in the body will cause movement in that part of the brain where the fiber is attached. The variety of different movements of the animals spirits cause a variety of different sensations not in the part of the body originally affected but only in the brain and ultimately in the pineal gland. So, strictly speaking, pain does not occur in the foot when a toe is stubbed but only in the brain. This, in turn, may cause the widening or narrowing of pores in the brain so as to direct the animals spirits to various muscles and make them move. For example, the sensation of heat is produced by the imperceptible particles in the pot of boiling water, which caused the movement of the animal spirits in the nerves terminating at the end of the hand. These animal spirits then move the fibers extending to the brain through the tube of nerves causing the sensation of pain. This then causes various pores to widen or narrow in the brain so as to direct the animals spirits to the muscles of the arm and cause it to quickly move the hand away from the heat in order to remove it from harm. This is the model for how all sensations occur.

These sensations may also cause certain emotions or passions in the mind. However, different sensations do not give rise to different passions because of the difference in objects but only in regards to the various ways these things are beneficial, harmful or important for us. Accordingly, the function of the passions is to dispose the soul to want things that are useful and to persist in this desire Moreover, the same animal spirits causing these passions also dispose the body to move in order to attain them. For example, the sight of an ice cream parlor, caused by the movement of the animal spirits in the eye and through the nerves to the brain and pineal gland, might also cause the passion of desire to arise. These same animal spirits would then dispose the body to move (for example, toward the ice cream parlor) in order to attain the goal of eating ice cream thereby satisfying this desire. Descartes goes on to argue that there are only six primitive passions, namely wonder, love, hatred, desire, joy and sadness. All other passions are either composed of some combination of these primitives or are species of one of these six genera. Much of the rest of parts 2 and 3 of the  Passions of the Soul is devoted to detailed explications of these six primitive passions and their respective species.

10. Morality

A. the provisional moral code.

In Part 3 of the  Discourse on Method , Descartes lays out a provisional moral code by which he plans to live while engaged in his methodological doubt in search of absolute certainty. This code of “three or four” rules or maxims is established so that he is not frozen by uncertainty in the practical affairs of life. These maxims can be paraphrased as follows:

• To obey the laws and customs of my country, holding constantly to the Catholic religion, and governing myself in all other matters according to the most moderate opinions accepted in practice by the most sensible people.
• To be as firm and decisive in action as possible and to follow even the most doubtful opinions once they have been adopted.
• Try to master myself rather than fortune, and change my desires rather than the order of the world.
• Review the various professions and chose the best (AT VI 23-28: CSM I 122-125).

The main thrust of the first maxim is to live a moderate and sensible life while his previously held beliefs have been discarded due to their uncertainty. Accordingly, it makes sense to defer judgment about such matters until certainty is found. Presumably Descartes defers to the laws and customs of the country in which he lives because of the improbability of them leading him onto the wrong path while his own moral beliefs have been suspended. Also, the actions of sensible people, who avoid the extremes and take the middle road, can provide a temporary guide to action until his moral beliefs have been established with absolute certainty. Moreover, although Descartes does seems to bring his religious beliefs into doubt in the  Meditations , he does not do so in the  Discourse . Since religious beliefs can be accepted on faith without absolutely certain rational justification, they are not subject to methodological doubt as employed in the  Discourse . Accordingly, his religious beliefs can also serve as guides for moral conduct during this period of doubt. Therefore, the first maxim is intended to provide Descartes with guides or touchstones that will most likely lead to the performance of morally good actions.

The second maxim expresses a firmness of action so as to avoid the inaction produced by hesitation and uncertainty. Descartes uses the example of a traveler lost in a forest. This traveler should not wander about or even stand still for then he will never find his way. Instead, he should keep walking in a straight line and should never change his direction for slight reasons. Hence, although the traveler may not end up where he wants, at least he will be better off than in the middle of a forest. Similarly, since practical action must usually be performed without delay, there usually is not time to discover the truest or most certain course of action, but one must follow the most probable route. Moreover, even if no route seems most probable, some route must be chosen and resolutely acted upon and treated as the most true and certain. By following this maxim, Descartes hopes to avoid the regrets experienced by those who set out on a supposedly good course that they later judge to be bad.

The third maxim enjoins Descartes to master himself and not fortune. This is based on the realization that all that is in his control are his own thoughts and nothing else. Hence, most things are out of his control. This has several implications. First, if he has done his best but fails to achieve something, then it follows that it was not within his power to achieve it. This is because his own best efforts were not sufficient to achieve that end, and so whatever effort would be sufficient is beyond his abilities. The second implication is that he should desire only those things that are within his power to obtain, and so he should control his desires rather than try to master things beyond his control. In this way, Descartes hopes to avoid the regret experienced by those who have desires that cannot be satisfied, because this satisfaction lies beyond their grasp so that one should not desire health when ill nor freedom when imprisoned.

It is difficult to see why the fourth maxim is included. Indeed, Descartes himself seems hesitant about including it when he states at the outset that his provisional moral code consists of “three or four maxims.” Although he does not examine other occupations, Descartes is content with his current work because of the pleasure he receives from discovering new and not widely known truths. This seems to imply the correct choice of occupation can ensure a degree of contentedness that could not be otherwise achieved if one is engaged in an occupation for which one is not suited. Descartes also claims that his current occupation is the basis of the other three maxims, because it is his current plan to continue his instruction that gave rise to them. He concludes with a brief discussion of how his occupational path leads to the acquisition of knowledge, which, in turn, will lead to all the true goods within his grasp. His final point is that learning how best to judge what is good and bad makes it possible to act well and achieve all attainable virtues and goods. Happiness is assured when this point is reached with certainty.

b. Generosity

After the  Discourse of 1637, Descartes did not take up the issue of morality in any significant way again until his correspondence with Princess Elizabeth in 1643, which culminated in his remarks about generosity in part 3 of the  Passions of the Soul . Given the temporal distance between his main reflections on morality, it is easy to attribute to Descartes two moral systems – the provisional moral code and the ethics of generosity. But Descartes’ later moral thinking retains versions of the second and third maxim without much mention of the first and fourth. This indicates that Descartes’ later moral theory is really an extension of his earlier thought with the second and third maxims at its core. At  Passions , part 3, section 153, Descartes claims that the virtue of generosity “causes a person’s self-esteem to be as great as it may legitimately be” and has two components. First is knowing that only the freedom to dispose volitions is in anyone’s power. Accordingly, people should only be praised or blamed for using one’s freedom either well or poorly. The second component is the feeling of a “firm and constant resolution” to use one’s freedom well such that one can never lack the will to carry out whatever has been judged to be best.

Notice that both components of generosity relate to the second and third maxim of the earlier provisional moral code. The first component is reminiscent of the third maxim in its acknowledgment of people’s freedom of choice and the control they have over the disposition of their will or desire, and therefore they should be praised and blamed only for those things that are within their grasp. The second component relates to the second maxim in that both pertain to firm and resolute action. Generosity requires a resolute conviction to use free will correctly, while the second maxim is a resolution to stick to the judgment most likely to lead to a good action absent a significant reason for changing course. However, a difference between these two moral codes is that the provisional moral code of the  Discourse focuses on the correct use and resolute enactment of probable judgments, while the later ethics of generosity emphasizes a firm resolution to use free will correctly. Hence, in both moral systems, the correct use of mental faculties, namely judgment and free will, and the resolute pursuit of what is judged to be good is to be enacted. This, in turn, should lead us to a true state of generosity so as to legitimately esteem ourselves as having correctly used those faculties through which humans are most in the likeness of God.

11. References and Further Reading

A. primary sources.

• This is still the standard edition of all of Descartes’ works and correspondence in their original languages. Cited in the text as AT volume, page.
• This is the standard English translation of Descartes philosophical works and correspondence. Cited in the text as CSM or CSMK volume, page.

b. Secondary Sources

• This is a collection of essays by prominent scholars about various issues raised in the  Meditations , objections to them and the adequacy or inadequacy of Descartes’ replies.
• A study of Descartes’ method and its results.
• A clear and concise introduction to Descartes’ philosophy.
• A classic examination of Descartes’  Meditations .
• Provides a detailed account of Cartesian science and its metaphysical foundations.
• Though somewhat technical, this is a very good biography of Descartes’ intellectual development emphasizing his early years and his interests in mathematics and science.
• A classic study of Descartes’ philosophy through the  Meditations .
• One of the few book length explications of Descartes’ moral theory.
• This is a very readable and enjoyable biography.
• Provides an interpretation of the real distinction between mind and body, their causal interaction and theory of sensation within the context of late Scholastic theories of soul-body union and sensation.
• An at times technical, though readable, account of the whole of Descartes’ metaphysics from within the context of late Scholasticism.
• Provides an account of Descartes’ theory of mind-body union and how it helps him to avoid the mind-body problem.
• Provides a history and account of the controversies at Utrecht and Leiden.
• An anthology of essays by many noted scholars on Descartes’ theory of the passions and aspects of his later moral theory.
• Classic account of Descartes’ philosophy in general.
• A classic in Descartes scholarship covering the whole of his philosophy as expressed in the  Meditations .

Author Information

Justin Skirry Email:  [email protected] Nebraska-Wesleyan University U. S. A.

An encyclopedia of philosophy articles written by professional philosophers.

 MacTutor

René descartes.

I entirely abandoned the study of letters, resolving to seek no knowledge other than that which could be found in myself or else in the great book of the world. I spent the rest of my youth travelling, visiting courts and armies, mixing with people of diverse temperaments and ranks, gathering various experiences, testing myself in the situations which fortune offered me, and at all times reflecting on whatever came my way so as to derive some profit from it.

[ I want to promote a ] completely new science by which all questions in general may be solved that can be proposed about any kind of quantity, continuous as well as discrete. But each according to its own nature. ... In arithmetic, for instance, some questions can be solved by rational numbers, some by surd numbers, and others can be imagined but not solved. For continuous quantity I hope to prove that, similarly, certain problems can be solved by using only straight or circular lines, that some problems require other curves for their solution, but still curves which arise from one single motion and which therefore can be traced by the new compasses, which I consider to be no less certain and geometrical than the usual compasses by which circles are traced; and, finally, that other problems can be solved by curved lines generated by separate motions not subordinate to one another.

[ The foundations of physics ] is the topic which I have studied more than any other and in which, thank God, I have not altogether wasted my time. At least I think that I have found how to prove metaphysical truths in a manner which is more evident than the proofs of geometry - in my opinion, that is: I do not know if I shall be able to convince others of it. During my first nine months in this country I worked on nothing else.

... in order to express my judgement more freely, without being called upon to assent to, or to refute the opinions of the learned, I resolved to leave all this world to them and to speak solely of what would happen in a new world, if God were now to create ... and allow her to act in accordance with the laws He had established.

As throughout his life he continued to do his work abed in the mornings. His evenings he generally devoted to the consideration of his correspondence, which was mainly scientific, rarely personal, and of which he was painstakingly careful, while the intermediate part of the day he gave to relaxation. In matters of money he was neither extravagant nor parsimonious, showing himself in this respect a true philosopher. He always did some entertaining, now more, now less, professing to find considerable enjoyment in conversation, though he was himself rather taciturn.

I have tried in my "Dioptrique" and my "Météores" to show that my Méthode is better than the vulgar, and in my "Géométrie" to have demonstrated it.

... and we see by experience that water which has been kept on a fire for some time freezes more quickly than otherwise, the reason being that those of its parts which can be most easily folded and bent are driven off during the heating, leaving only those which are rigid.

• He makes the first step towards a theory of invariants, which at later stages derelativises the system of reference and removes arbitrariness .
• Algebra makes it possible to recognise the typical problems in geometry and to bring together problems which in geometrical dress would not appear to be related at all .
• Algebra imports into geometry the most natural principles of division and the most natural hierarchy of method .
• Not only can questions of solvability and geometrical possibility be decided elegantly, quickly and fully from the parallel algebra, without it they cannot be decided at all .

... the Praxis was read by Descartes, and every line of Descartes' analysis bears token of the impression.

Thus entrenched as the Cartesian system was ... it was not to be wondered at that the pure and sublime doctrines of the 'Principia' were distrustfully received ... The uninstructed mind could not readily admit the idea that the great masses of the planets were suspended in empty space, and retained their orbits by an invisible influence ...

I should look upon Des-Cartes as a man most truly inspired in the knowledge of Nature, than any that have professed themselves so these sixteen hundred years...

Why are not your vortices in the form of columns or cylinders rather than ellipses, since any point of the axis of a vortex is as it were a centre from which the celestial matter recedes with, as far as I can see, a wholly constant impetus? ... Who causes all the planets not to revolve in one plane ( the plane of the ecliptic ) ? ... And the Moon itself, neither in the plane of the Earth's equator nor in a plane parallel to this?

I would not value these Rules so highly if they were good only for solving those pointless problems with which arithmeticians and geometers are inclined to while away their time, for in that case all I could credit myself with achieving would be to dabble in trifles with greater subtlety than they. I shall have much to say below about figures and numbers, for no other disciplines can yield illustrations as evident and certain as these. But if one attends closely to my meaning, one will readily see that ordinary mathematics is far from my mind here, that it is quite another discipline I am expounding, and that these illustrations are more its outer garments than its inner parts. This discipline should contain the primary rudiments of human reason and extend to the discovery of truths in any field whatever. Frankly speaking, I am convinced that it is a more powerful instrument of knowledge than any other with which human beings are endowed, as it is the source of all the rest.

In appearance Descartes was a small man of rather slight figure with a large head. His nose was prominent, his lower lip somewhat protruding, his beard and moustache of a semi-military type, and his hair growing down upon his forehead almost to his eyebrows. He wore a wig of natural colour to which he always gave fastidious attention, as he did also to his clothes which were now invariably of black cloth. In demeanour he was generally cheerful, rarely gay. His manners were always refined, gentle, and polite, and his temper tranquil and easy. As a personality he was proud, somewhat aristocratically reserved, sensitive, a bit angular, and, though a shade domineering, was pre-eminently obliging.

He always was well dressed, and wore a sword. He was not industrious; he worked short hours, and read little. When he went to Holland he took few books with him, but among them were the Bible and Thomas Aquinas. His work seems to have been done with great concentration during short periods; but perhaps, to keep up the appearance of a gentlemanly amateur, he may have pretended to work less than in fact he did, for otherwise his achievements seem scarcely credible.

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Encyclopedia of Humanities

The most comprehensive and reliable Encyclopedia of Humanities

René Descartes

We explain who René Descartes was, where he was born, his works and main contributions. In addition, we explore his main characteristics and most famous quotations.

Who was René Descartes?

René Descartes (1596-1650) was a French philosopher, scientist and mathematician , traditionally regarded as the father of modern philosophy, analytic geometry and mechanistic physics.

Descartes was one of the great scientific minds of his time and remains a seminal figure in Western culture today . He is considered an innovator and a disruptor of the methods and theories accepted by the academy at the time, which he helped to rebuild.

Many philosophical works and concepts based on the postulates of Descartes bear the adjective "Cartesian" in reference to the thinker. Whether in metaphysics, theology or epistemology , Descartes is a figure constantly revisited, which is proof that his thought still speaks to us now and continues to exert a profound influence on present-day philosophers.

Among his numerous and varied works are fundamental titles such as Rules for the Direction of the Mind (1628), Discourse on the Method (1637) and Metaphysical Meditations (1641).

• See also: Pythagoras

Life and Education of Descartes

Descartes was born on March 13, 1596 in Touraine (France) , nowadays called "Descartes" in his honor. He was the third child of a French family of the lower nobility. His father, Joachim Descartes, was a member of the Parliament of Brittany and his mother, Jeanne Brochard, was the daughter of the mayor of Nantes. At the time of his birth his parents were fleeing Rennes to escape the bubonic plague. Only a few months old, Descartes was orphaned of his mother on May 13, 1597 and was left in the care of his father at his maternal grandmother’s home.

Descartes received early education at the Jesuit Collège Royal Henri-Le-Grand at La Flèche , from the age of eleven to sixteen. Even if he had to follow a special diet for his alleged delicate health, he was highly appreciated by the educational community. From an early age he learned and excelled in physics, scholastic philosophy and mathematics. He also studied Greek and Latin and read authors such as Aristotle , Cicero, Homer and Plato , among others.

In 1614 he entered the University of Poitiers, where he studied medicine and law, earning a Baccalauréat (bachelor's degree) and a Licence (master's degree) . After a brief incursion into military life and after having traveled to Denmark and Germany, Descartes returned to France and settled in Paris. He remained there until 1629, when he settled permanently in the Netherlands.

In 1649 Queen Christina of Sweden summoned him to Stockholm , where he died of pneumonia in February of the following year. The cause of his death is often a subject of speculation. Eike Pies, German historian and physician, argues in his book The Murder of Descartes that he was murdered with arsenic poisoning.

Fields of interest of René Descartes

Descartes had a preference for the sciences that flourished during and after the Renaissance, considering himself a Copernican and an atomist , following the studies of Copernicus, Galileo, and Gassendi.

He maintained close correspondence with many thinkers of his time, engaging in frequent discussions on physics, mathematics, and philosophy, his three major fields of work.

His works encompass a wide range of fields . Examples of these are his contributions to geometry, optics, physics, mathematics and philosophy in most of its branches.

Works by René Descartes

Except for a few notes from his youth, Descartes' first surviving work was his Rules for the Direction of the Mind (1701) , which was written in 1628 but published posthumously. Other important works published during his lifetime include Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences, Meditations on First Philosophy, The Search for Truth by Natural Reason and Principles of Philosophy, among others.

Most of his works were written in Latin, as was customary at the time although many were also written in his native language, French. Some of his most fundamental works considered central to Western philosophy continue to be standard texts at universities and schools of philosophy today. These include:

• Discourse on the Method (1637) . Originally written in French, it is divided into six parts and narrates Descartes' life story and the circumstances he encountered in order to find a unitary method of knowledge. Here appears the Cartesian or hyperbolic doubt for the first time, which Descartes elaborates. He goes on to expound on the four principles to be followed to arrive at the ultimate truth of things. Truth is characterized as distinct and evident, the two criteria for distinguishing it.
• Meditations on First Philosophy (1641) . Originally written in French, the meditations present the philosophical system introduced by Descartes in his Discourse on the Method . Divided into six parts (each one a meditation), the book is a metaphysical elaboration of the Cartesian method of hyperbolic doubt. After an initial meditation in which Descartes rejects the sources of all possible knowledge (the senses and reason), the other five meditations revolve around the possibility of restoring the validity of those same sources to conclude that humans are not always mistaken and can arrive at a distinct and evident truth.

Main ideas of René Descartes

Descartes' philosophical works marked the transition from one era (the medieval world) to philosophical modernity . Most of his works revolve around the criticism of established methods of thought, the construction of a new method to attain truth, the development of hyperbolic doubt (the "methodic doubt") and the ego cogito as the first evident truth.

• Deduction and induction . The method consisted in the application of deductive and inductive reasoning of science and philosophy. These reasonings rejected the Scholastic doctrine, which relied on the views of Ancient masters and on the canonical interpretation of the Scriptures to guarantee truth.
• The existence of God . Though his thought acknowledged the existence of God, many scholars argue that God only appears to avoid problems with the Church. It is undeniable that Descartes lived at a time of censorship and intellectual persecution (as happened with Galileo). However, in the Meditations on First Philosophy, God as the guarantor of truth plays such a crucial role in recovering the world that his presence is inevitable.
• Methodic doubt . Also known as "hyperbolic" doubt (due to its exaggerated nature) or "Cartesian" doubt (in reference to Descartes). Doubt is proposed as a methodological approach in his Discourse on the Method and is later put into practice in Meditations on First Philosophy . Descartes argues that anything that may be subject to doubt must be discarded as certain. Cartesian doubt means to know beyond not merely all reasonable doubt but all possible doubt.
• Ego cogito . Although various precedents can be traced, Descartes is credited as the first to formulate the ego cogito : " I think, therefore I am ". This dictum, its meaning, and its utility in the Meditations are still central in philosophical discussions today. Broadly speaking, the ego cogito affirms the presence of the self above all things. The world as extended matter ( res extensa ), the body and even the senses play a secondary role compared to the evidence and inevitability of the Cartesian ego. Even in doubt, deception or mistake, the existence of the self is indubitable and necessary in order to experience each of these.

Other philosophical ideas and concepts introduced by Descartes include the mind-body dualism, the immortality of the soul, the immanence of ideas, the levels of reality, the physical functioning of the body (which is very close to contemporary understanding) and the origin of the content of dreams, among others.

Contributions of René Descartes in other fields

Descartes’ main contribution to mathematics was the emergence of analytic geometry , since he connected the previously separate fields of geometry and algebra, as well as the formulation of the theory of equations. His contributions to the field were numerous and are related to his method of formulating concepts.

For instance, he introduced the use of the letters of the alphabet as variables , making the distinction between the first letters (A, B, C...) for the known and the last letters (X, Y, Z...) for the unknown.

Furthermore, Descartes introduced the mathematical notation used to indicate powers or exponents and the Cartesian Rule of Signs . Today, we speak of  "Cartesian plane" in his honor.

While his contributions to physics were not as significant, optics and mechanics greatly benefited from his thinking . By replacing spiritual values in the works of earlier thinkers with mechanical interpretations, Descartes was able to better approach physical phenomena and thus laid the foundations for the modern scientific method.

Recognition of René Descartes

Descartes' remains were exhumed in 1676 and placed in a copper coffin to be transported to Paris, where they were initially located in the Church of Saint-Geneviève-du-Mont, then in the Panthéon and finally in the Abbey of Saint-Germain-des-Prés. His skull is currently housed in the Museum of Man in the same city. His hometown as well as one of the lunar craters bear his name.

• Cottingham, J. G. (1993). A Descartes dictionary . Blackwell Reference.
• Cottingham, J. G. (1995). Descartes . Universidad Nacional Autónoma de México.
• Cottingham, J. G. (1992). The Cambridge Companion to Descartes . Cambridge University Press.

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Encyclopedia of Renaissance Philosophy pp 1–10 Cite as

Descartes, René

Born : 31 March 1596 (La Haye en Touraine, France)

Died : 11 February 1650 (Stockholm, Sweden)

• Fabrizio Baldassarri 2  
• Living reference work entry
• First Online: 15 April 2020

110 Accesses

Scholars generally consider René Descartes to be the father of early modern philosophy insofar he rejected scholasticism, Aristotelianism, but also Renaissance philosophies and grounded a new system of knowledge whose roots lie solely in the mind and not in previous assumptions. This defines a modern self that achieves the knowledge of nature and shapes the modern universe. In the Discours de la Méthode (1637) and the Meditationes de prima philosophia (1641), after rejecting earlier education, doctrines, and scientiæ , Descartes isolates the powers of the mind (i.e., clear and distinct ideas) as the beginning of any certainty. Yet, having considered Descartes’ system as dismissing all doctrines and beliefs, in this entry I will first examine some relations between the French philosopher and those contexts and then discuss the novelties of his system. After a short biography, in the second section, I will briefly explore the interrelations between Descartes and Renaissance scholars, whom he reproached for their precipitate conclusions. Third, I will unearth his criticism of Aristotelian-scholastic philosophy (which he reproached for its preconceptions), while highlighting his attention to a few Aristotelian texts. Fourth, I will investigate a few innovative aspects of his methodology; this consists of novel combination of intellectual cognition and experimentation. As a result, Descartes’ entire natural philosophy consists of a theoretical framework that defines the principles of knowledge and the architecture of science, while the body of all disciplines and the knowledge of particular issues are methodologically and experientially constructed. Despite several limitations, Descartes’ system is remarkably innovative.

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Primary Literature

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Baldassarri, F. (2020). Descartes, René. In: Sgarbi, M. (eds) Encyclopedia of Renaissance Philosophy. Springer, Cham. https://doi.org/10.1007/978-3-319-02848-4_645-1

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René Descartes

René Descartes (1596–1650) was a creative mathematician of the first order, an important scientific thinker, and an original metaphysician. During the course of his life, he was a mathematician first, a natural scientist or “natural philosopher” second, and a metaphysician third. In mathematics, he developed the techniques that made possible algebraic (or “analytic”) geometry. In natural philosophy, he can be credited with several specific achievements: co-framer of the sine law of refraction, developer of an important empirical account of the rainbow, and proposer of a naturalistic account of the formation of the earth and planets (a precursor to the nebular hypothesis). More importantly, he offered a new vision of the natural world that continues to shape our thought today: a world of matter possessing a few fundamental properties and interacting according to a few universal laws. This natural world included an immaterial mind that, in human beings, was directly related to the brain; in this way, Descartes formulated the modern version of the mind–body problem. In metaphysics, he provided arguments for the existence of God, to show that the essence of matter is extension, and that the essence of mind is thought. Descartes claimed early on to possess a special method, which was variously exhibited in mathematics, natural philosophy, and metaphysics, and which, in the latter part of his life, included, or was supplemented by, a method of doubt.

Descartes presented his results in major works published during his lifetime: the Discourse on the Method (in French, 1637), with its essays, the Dioptrics , Meteorology , and Geometry ; the Meditations on First Philosophy (i.e., on metaphysics), with its Objections and Replies (in Latin, 1641); the Principles of Philosophy , covering his metaphysics and much of his natural philosophy (in Latin, 1644); and the Passions of the Soul , on the emotions (in French, 1649). Important works published posthumously included his Letters (in Latin and French, 1657–67); World, or Treatise on Light , containing the core of his natural philosophy (in French, 1664); Treatise on Man (in French, 1664), containing his physiology and mechanistic psychology; and the Rules for the Direction of the Mind (in Latin, 1704), an early, unfinished work attempting to set out his method.

Descartes was known among the learned in his day as the best of the French mathematicians, as the developer of a new physics, and as the proposer of a new metaphysics. In the years following his death, his natural philosophy was widely taught and discussed. In the eighteenth century aspects of his science remained influential, especially his physiology, and he was remembered for his failed metaphysics and his method of doubt. In the nineteenth century he was revered for his mechanistic physiology and theory that animal bodies are machines (that is, are constituted by material mechanisms, governed by the laws of matter alone). The twentieth century variously celebrated his famous “cogito” starting point, reviled the sense data that some alleged to be the legacy of his skeptical starting point, and looked to him as a model of the culturally engaged philosopher. He has been seen, at various times, as a hero and as a villain; as a brilliant theorist who set new directions in thought, and as the harbinger of a cold, rationalistic, and calculative conception of human beings.

1.1 Early life and education

1.2 first results, a new mission, and method, 1.3 metaphysical turn, comprehensive physics, discourse, 1.4 the metaphysics and comprehensive physics revealed, 1.5 theological controversy, passions , and death, 2. philosophical development, 3.1 how do our minds know, 3.2 the mark of truth, 3.3 the nature of reality, 3.4 mind–body relation, 3.5 god and error, 4. the new science, 5. theory of sense perception, english translations, other readings, other internet resources, related entries, 1. intellectual biography.

Descartes was born on 31 March 1596 in his maternal grandmother's house in La Haye, in the Touraine region of France. His father Joachim, a lawyer who lived in Châtteleraut (22 kilometers southwest of La Haye, across the Creuse River in the Poitou region), was away at the Parliament of Brittany in Rennes. The town of La Haye, which lies 47 kilometers south of Tours, has subsequently been renamed Descartes.

When Descartes was thirteen and one-half months old, his mother, Jeanne Brochard, died in childbirth. The young René spent his first years with his grandmother, Jeanne Sain Brochard, in La Haye, together with his older brother Pierre and older sister Jeanne. It is likely that he then moved to the house of his great uncle, Michel Ferrand, who, like many of René's male relatives, was a lawyer, and who was Counselor to the King in Châtteleraut. When Descartes met Isaac Beeckman in 1618, Descartes introduced himself as “Poitevin,” or from Poitou ( 10:46, 51–4 ; Rodis-Lewis 1998, 26; see also 2:642 ). At this time (and now and again later on), he signed letters as “du Perron” and called himself “sieur du Perron” (Lord of Perron), after a small farm in Poitou he had inherited from his mother's family (Watson 2007, 81, 230). But he did not neglect his birth place in La Haye: in a letter of 1649, he described himself as “a man who was born in the gardens of Touraine” (5:349).

In 1606 or 1607, Descartes entered the newly founded Jesuit College of La Fleche, where he remained until 1614 or 1615. He followed the usual course of studies, which included five or six years of grammar school, including Latin and Greek grammar, classical poets, and Cicero, followed by three years of philosophy curriculum. By rule, the Jesuit curriculum was based on the philosophy of Aristotle, and divided into the then-standard topics of logic, morals, physics, and metaphysics. The Jesuits also included mathematics in the final three years of study.

Aristotle's philosophy was approached through textbooks and commentaries. Even within this framework, and taking into account the reading of Cicero, Descartes would have been exposed to the doctrines of the ancient atomists, Plato, and the Stoics, and he would have heard of the skeptics. Further, important intellectual events were known at La Fleche, including the discovery of the moons of Jupiter by Galileo in 1610.

Famously, Descartes wrote in the autobiographical portion of the Discourse that, when he left school, “I found myself beset by so many doubts and errors that I came to think I had gained nothing from my attempts to become educated but increasing recognition of my ignorance” (6:4). And yet in the next paragraph he allowed that he did not “cease to value the exercises done in the schools” (6:5), for languages, fables, oratory, poetry, mathematics, morals, theology, and philosophy all had their value, as did jurisprudence, medicine, and the other sciences (including engineering), which serve as professions and which one might study after attending a school such as La Fleche. He went on to note the contradiction and disagreement that beset philosophy and so infected the higher sciences (including medicine) “insofar as they borrow their principles from philosophy” (6:8). A year later, in 1638, he advised a correspondent on the virtues of sending the correspondent's son to La Fleche (even if he wanted his son subsequently to transcend the learning of the schools), while also suggesting that the son might study at Utrecht with Henry le Roy, a disciple of Descartes (2:378–9). Descartes was, in the Discourse , suggesting that it was no accident that the philosophy he learned at La Fleche was uncertain: previous philosophy was bound to be uncertain, since he (Descartes) was now offering a first glimpse of the one true philosophy that he had only recently discovered. Until it could be promulgated, La Fleche, or another good school, would be the best on offer.

His family wanted Descartes to be a lawyer, like his father and many other relatives. To this end, he went to Poitiers to study law, and obtained a degree in 1616. But he never practiced law or entered into the governmental service such practice would make possible. Instead, he became a gentleman soldier, moving in 1618 to Breda, to support the Protestant Prince Maurice against the Catholic parts of the Netherlands (later, Belgium), which were controlled by Spain—a Catholic land, like France, but at this point an enemy.

While in Breda, Descartes met Isaac Beeckman, a Dutch mathematician and natural philosopher. Beeckman set various problems for Descartes, including questions about falling bodies, hydrostatics, and mathematical problems. Descartes and Beeckman engaged in what they called “physico-mathematica,” or mathematical physics (10:52) . Since antiquity, mathematics had been applied to various physical subject matters, in optics, astronomy, mechanics (focusing on the lever), and hydrostatics. Beeckman and Descartes brought to this work a commitment to atoms as the basic constituents of matter; as had ancient atomists, they attributed not only size, shape, and motion but also weight to those atoms (10:68) . Descartes opened a section in his notebook entitled “Democritica” (10:8) , in honor of the ancient atomist Democritus.

At this time, Descartes discovered and conveyed to Beeckman the fundamental insight that makes analytic geometry possible: the technique for describing lines of all sorts by using mathematical equations involving ratios between lengths. Descartes himself did not foresee replacing geometrical constructions with algebraic formulas; rather, he viewed geometry as the basic mathematical science and he considered his algebraic techniques to provide a powerful alternative to actual compass-and-ruler constructions when the latter became too intricate. When, in the nineteenth century, algebra and analysis took precedence over geometry, the rectilinear coordinate system of algebraic geometry came to be called “Cartesian coordinates” in honor of Descartes' discovery.

Descartes left Breda in 1619 to join the Catholic army of Maximilian I (Duke of Bavaria and ally of France). The war concerned the authority of Ferdinand V, a Catholic, who had been crowned emperor of the Holy Roman Empire in September. Descartes attended the coronation and was returning to the army when winter caught him in the small town of Ulm (or perhaps Neuburg), not far from Munich. On the night of November 10, 1619, Descartes had three dreams that seemed to provide him with a mission in life. The dreams themselves are interesting and complex (see Sebba 1987). Descartes took from them the message that he should set out to reform all knowledge. He decided to begin with philosophy, since the principles of the other sciences must be derived from it (6:21–2).

Descartes' activities over the next nine years are not well documented. He lived part of the time in Paris, visited Poitou to sell some inherited properties, and also traveled to Italy. During this time he worked on some mathematical problems, and he derived the sine law of refraction (later published in the Dioptrics ). His major effort was on a work to convey his new method, the Rules .

In the Rules , he sought to generalize the methods of mathematics so as to provide a route to clear knowledge of everything that human beings can know. His methodological advice included a suggestion that is familiar to every student of elementary geometry: break your work up into small steps that you can understand completely and about which you have utter certainty, and check your work often. But he also had advice for the ambitious seeker of truth, concerning where to start and how to work up to greater things. Thus, Rule 10 reads: “In order to acquire discernment we should exercise our intelligence by investigating what others have already discovered, and methodically survey even the most insignificant products of human skill, especially those which display order” (10:403). As examples of “simple” arts “in which order prevails,” he offered carpet-making and embroidery, and also number-games and arithmetic games. He went on to discuss the roles of the “cognitive faculties” in acquiring knowledge, which include the intellect, imagination, sense perception, and memory. These faculties allow the seeker of knowledge to combine simple truths in order to solve more complex problems, such as the solution to problems in optics (10:394), or the discovery of how a magnet works (10:427).

By the end of 1628 Descartes had abandoned work on the Rules , having completed about half of the projected treatise. In that year he moved to the Dutch Netherlands, and after that he returned to France infrequently, prior to moving to Sweden in 1649. While in the Netherlands, he endeavored to keep his address a secret and he changed locations frequently, in accordance with his motto, “to live well, live secretly.”

Upon arriving in the Netherlands, Descartes undertook work on two sorts of topics. In Summer, 1629, an impressive set of parhelia, or false suns, were observed near Rome. When Descartes heard of them, he set out to find an explanation. (He ultimately hypothesized that a large, solid ice-ring in the sky acts as a lens to form multiple images of the sun [6:355].) This work interrupted his investigations on another topic, which had engaged him for his first nine months in the Netherlands (1:44)—the topic of metaphysics, that is, the theory of the first principles of everything that there is. The metaphysical objects of investigation included the existence and nature of God and the soul (1:144, 182). However, these metaphysical investigations were not entirely divorced from problems such as the parhelia, for he claimed that through his investigations into God and the human self, he had been able “to discover the foundation of physics” (1:144). Subsequently, Descartes mentioned a little metaphysical treatise in Latin—presumably an early version of the Meditations —that he wrote upon first coming to the Netherlands (1:184, 350). And we know that Descartes later confided to Mersenne that the Meditations contained “all the principles of my physics” (3:233).

While working on the parhelia, Descartes conceived the idea for a very ambitious treatise. He wrote to Mersenne that he had decided not to explain “just one phenomenon” (the parhelia), but rather to compose a treatise in which he explained “all the phenomena of nature, that is to say, the whole of physics” (1:70). This work eventually became The World , which was to have had three parts: on light (a general treatise on visible, or material, nature), on man (a treatise of physiology), and on the soul. Only the first two survive (and perhaps only they were ever written), as the Treatise on Light and Treatise on Man . In these works, which Descartes decided to suppress upon learning of the condemnation of Galileo (1:270, 305), he offered a comprehensive vision of the universe as constituted from a bare form of matter having only length, breadth, and depth (three-dimensional volume) and carved up into particles with size and shape, which may be in motion or at rest, and which interact through laws of motion enforced by God (11:33–4). These works contained a description of the visible universe as a single physical system in which all its operations, from the formation of planets and the transmission of light from the sun, to the physiological processes of human and nonhuman animal bodies, can be explained through the mechanism of moving matter arranged into shapes and structures and moving according to three laws of motion.

After suppressing his World , Descartes decided to put forward, anonymously, a limited sample of his new philosophy, in the Discourse with its attached essays. The Discourse recounted Descartes' own life journey, explaining how he had come to the position of doubting his previous knowledge and seeking to begin afresh. It offered some initial results of his metaphysical investigations, including mind–body dualism. It did not, however, engage in the deep skepticism of the later Meditations , nor did it claim to establish, metaphysically, that the essence of matter is extension. This last conclusion was presented merely as a hypothesis whose fruitfulness could be tested and proven by way of its results, as contained in the attached essays on Dioptrics and Meteorology . The latter subject area comprised “atmospheric” phenomena. In his Meteorology , Descartes described his general hypothesis about the nature of matter, before continuing on to provide accounts of vapors, salt, winds, clouds, snow, rain, hail, lightning, the rainbow, coronas, and parhelia.

Near the beginning of the Meteorology , Descartes wrote that he was working from the following “supposition” or hypothesis: “that the water, earth, air, and all other such bodies that surround us are composed of many small parts of various shapes and sizes, which are never so properly disposed nor so exactly joined together that there do not remain many intervals around them; and that these intervals are not empty but are filled with that extremely subtle matter through the mediation of which, I have said above, the action of light is communicated” (6:233). He presented a corpuscularian basis for his physics, which denied the atoms-and-void theory of ancient atomism and affirmed that all bodies are composed from one type of matter, which is infinitely divisible (6:239). In the World , he had presented his non-atomistic corpuscularism, but without denying void space outright and without affirming infinite divisibility (11:12–20).

In the Meteorology , he also proclaimed that his natural philosophy had no need for the “substantial forms” and “real qualities” that other philosophers “imagine to be in bodies” (6:239). He had taken the same position in the World , where he said that in conceiving his new “world” (i.e., his conception of the universe), “I do not use the qualities called heat, cold, moistness, and dryness, as the Philosophers do” (11:25). Indeed, Descartes claimed that he could explain these qualities themselves through matter in motion (11:26), a claim that he repeated in the Meteorology (6:235–6). In effect, he was denying the then-dominant scholastic Aristotelian ontology, which explained all natural bodies as comprised of a “prime matter” informed by a “substantial form,” and which explained qualities such as hot and cold as really inhering in bodies in a way that is “similar” to the qualities of hot and cold as we experience them tactually.

Unlike Descartes' purely extended matter, which can exist on its own having only size and shape, scholastic Aristotelians held that prime matter cannot exist on its own. To form a substance, or something that can exist by itself, prime matter must be “informed” by a substantial form (a form that renders something into a substance). The four Aristotelian elements, earth, air, fire, and water, had substantial forms that combined the basic qualities of hot, cold, wet, and dry: earth is cold and dry; air is hot and wet; fire is hot and dry; and water is cold and wet. These elements can themselves then serve as “matter” to higher substantial forms, such as the form of a mineral, or a magnet, or a living thing. Whether in the case of earth or of a living rabbit, the “form” of a thing directs its characteristic activity. For earth, that activity is to approach the center to the universe; water has the same tendency, but not as strongly. For this reason, Aristotelians explained, the planet earth has formed at the center, with water on its surface. A new rabbit is formed when a male rabbit contributes, through its sperm, the “form” of rabbithood to the seed-matter of the female rabbit. This form then organizes that matter into a the shape of a rabbit, including organizing and directing the activity of its various organs and physiological processes. The newborn rabbit's behavior is then guided by its rabbit-specific “sensitive soul,” which is the name for the substantial form of the rabbit. Other properties of the rabbit, such as the whiteness of its fur, are explained by the “real quality” of white inhering in each strand of hair.

Although in the World and Meteorology Descartes avoided outright denial of substantial forms and real qualities, it is clear that he intended to deny them (1:324; 2:200; 3:420, 500, 648). Two considerations help explain his tentative language: first, when he wrote these works, he was not yet prepared to release his metaphysics, which would support his hypothesis about matter and so rule out substantial forms (1:563); and, second, he was sensitive to the prudential value of not directly attacking the scholastic Aristotelian position (3:298), since it was the accepted position in university education (3:577) and was strongly supported by orthodox theologians, both Catholic and Protestant (1:85–6; 3:349).

After publication of the Discourse in 1637, Descartes received in his correspondence queries and challenges to various of the doctrines, including his account of the sequence of phenomena during heart-beat and the circulation of the blood; his avoidance of substantial forms and real qualities; his argument for a distinction between mind and body; and his view that natural philosophical hypotheses could be “proven” through the effects that they explain (6:76). Descartes' correspondence from the second half of the 1630s repays close study, among other things for his discussions of hypothesis-confirmation in science, his replies to objections concerning his metaphysics, and his explanation that he had left the most radical skeptical arguments out of this work, since it was written in French for a wide audience (1:350, 561).

In 1635, Descartes fathered a daughter named Francine. Her mother was Descartes' housekeeper, Helena Jans. They lived with Descartes in the latter 1630s, until Francine's untimely death in September 1640. Descartes subsequently contributed a dowry for Helena's marriage in 1644 (Watson 2007, 188).

In a letter of 13 November 1639, Descartes wrote to Mersenne that he was “working on a discourse in which I try to clarify what I have hitherto written” on metaphysics (2:622). This was the Meditations , and presumably he was revising or recasting the Latin treatise from 1629. He announced to Mersenne a plan to put the work before “the twenty or thirty most learned theologians” before it was published. In the end, he and Mersenne collected seven sets of objections to the Meditations , which Descartes published with the work, along with his replies. Some objections were from theologians, one set was from the Jesuit philosopher Pierre Bourdin, and others were from Mersenne himself, the philosophers Pierre Gassendi and Thomas Hobbes, and the philosopher-theologian Antoine Arnauld.

As previously mentioned, Descartes considered the Meditations to contain the principles of his physics. But there is no Meditation labeled “principles of physics.” The principles in question, which are spread through the work, concern the nature of matter (that its essence is extension), the activity of God in creating and preserving the world, the nature of mind (that it is unextended, thinking substance), mind–body union and interaction, and the ontology of sensory qualities.

Once Descartes had presented his metaphysics, he felt free to proceed with the publication of his entire physics. However, he needed first to teach it to speak Latin (3:523), the lingua franca of the seventeenth century. He hatched a scheme to publish a Latin version of his physics (the Principles ) together with a scholastic Aristotelian work on physics, so that the comparative advantages would be manifest. For this purpose, he chose the Summa philosophiae of Eustace of St. Paul. That part of his plan never came to fruition. His intent remained the same: he wished to produce a book that could be adopted into the schools, even Jesuit schools such as La Fleche (3:233, 523). Ultimately, he had some success in having his physics taught in the Netherlands and England; but for the Catholic lands, teaching of his philosophy was dampened when his works were placed on the Index of Prohibited Books in 1663.

The Principles appeared in Latin in 1644, with a French translation following in 1647. Descartes added to the French translation an “Author's Letter” to serve as a preface. In the letter he explained important elements of his attitude toward philosophy, including the view that in matters philosophical one must reason through the arguments and evaluate them for one's self (9B:3). He also presented an image of the relations among the various parts of philosophy, in the form of a tree:

Thus the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principal ones, namely medicine, mechanics and morals. By “morals” I understand the highest and most perfect moral system, which presupposes a complete knowledge of the other sciences and is the ultimate level of wisdom. (9B:14)

The extant Principles offer metaphysics in Part I; the general principles of physics, in the form of his matter theory and laws of motion, are presented in Part II, as following from the metaphysics; Part III concerns astronomical phenomena; and Part IV covers the formation of the earth and seeks to explain the properties of minerals, metals, magnets, fire, and the like, to which are appended discussions of how the senses operate and a final discussion of methodological issues in natural philosophy. His intent had been also to explain the origins of plants and animals, human physiology, mind–body union and interaction, and the function of the senses. In the end, he had to abandon the discussion of plants and animals ( Princ . IV.188), but he included some discussion of mind–body union in his account of the senses.

From early in his correspondence with Mersenne, Descartes showed a concern to avoid becoming embroiled in theological controversy or earning the enmity of church authorities (1:85–6, 150, 271). In the end, he was drawn into theological controversy with Calvinist theologians in the Netherlands. In the latter 1630s, Henry le Roy, or Regius, a professor of medicine in Utrecht, taught Descartes' system of natural philosophy. Already by 1640, Gisbert Voetius, a theologian at Utrecht, expressed his displeasure over this to Mersenne (3:230). Controversy brewed, at first between Regius and Voetius, with Descartes advising the latter. Voetius, who was rector of the University, convinced the faculty senate to condemn Descartes' philosophy in 1642. He and his colleagues published two works (in 1642 and 1643) attacking Descartes' philosophy, to which Descartes himself responded by publishing a Letter to Voetius (1643). The controversy simmered through the mid 1640s. Descartes eventually had a falling out with Regius, who published a broadsheet or manifesto, in which Regius departed from Descartes' theory of the human mind. Descartes replied with his Comments on a Certain Broadsheet (1648).

In the mid 1640s, Descartes continued work on his physiological system, which he had pursued throughout the 1630s. He had his Treatise on Man recopied, and he began a new work, Description of the Human Body . During this period he corresponded with Princess Elisabeth, at first on topics in metaphysics stemming from her reading of the Meditations , and then on the passions and emotions. Eventually, he wrote the Passions of the Soul , which gave the most extensive account of his behavioral physiology to be published in his lifetime, and which contained a comprehensive and original theory of the passions and emotions. Portions of this work constitute what we have of Descartes' moral theory.

In 1649, Descartes accepted the invitation of Queen Christina of Sweden to join her court. At the Queen's request, he composed the Statutes of the Swedish Royal Academy. On the day he delivered them to her, he became ill. He never recovered. He died on 11 February 1650.

In general, it is rare for a philosopher's positions and arguments to remain the same across an entire life. This means that, in reading philosophers' works and reconstructing their arguments, one must pay attention to the place of each work in the philosophical development of the author in question. Readers of the philosophical works of Immanuel Kant are aware of the basic distinction between his critical and precritical periods. Readers of the works of G. W. Leibniz are also aware of his philosophical development, although in his case there is less agreement on how to place his writings into a developmental scheme.

Scholars have proposed various schemes for dividing Descartes' life into periods. I favor a relatively simple division between the era when mathematics provided the model for his method, and the period after the “metaphysical turn” of 1629, when his conception of the role of the intellect in acquiring knowledge changed, and when he came to conceive the truth of his special or particular hypotheses in natural philosophy as less than certain, and so as subject to the scheme of confirmation through consequences mentioned above. In effect, he adopted a hypothetico-deductive scheme of confirmation, but with this difference: the range of hypotheses was limited by his metaphysical conclusions concerning the essence of mind and matter, their union, and the role of God in creating the preserving the universe. Consequently, some hypotheses, such as the “substantial forms” of scholastics, were ruled out. I explain argumentative differences among the World , Discourse , and Meditations and Principles as stemming from the fact that in the 1630s Descartes was arguing without yet having presented his metaphysics, and so he adopted an empirical mode of justification, whereas after 1641 he could appeal to his published metaphysics in seeking to secure the general framework of his physics.

Other scholars see things differently. John Schuster (1980) finds that the epistemology of the Rules lasted into the 1630s, and was superseded (unhappily, in his view) only by the metaphysical quest for certainty of the Meditations . Daniel Garber (1992, 48) also holds that Descartes abandoned his early method after the Discourse . Machamer and McGuire (2006) believe that Descartes expected natural philosophy to meet the standard of absolute certainty through the time of the Meditations , and that he in effect admitted defeat on that score in the final articles of the Principles , adopting a lower standard of certainty for his particular hypotheses (such as the explanation of magnetism by cork-screw shaped particles). They see the Principles as marking Descartes' “epistemic turn” away from the methodological stance of realism found in the Rules , Discourse , and Meditations .

These contrasting views of Descartes' intellectual development suggest different relations between his metaphysics and physics. Schuster (1980) treats Descartes' metaphysical arguments as a kind of afterthought. Machamer and McGuire (2006) see Descartes' alleged “epistemic turn” and his retreat from realism as a response to philosophical criticism in 1641; they find more continuity between Descartes' Rules and his writings up to 1641 than do Garber or I. In the version I have presented, Descartes was working on physical problems first, but his metaphysical insights of 1628–9 allowed him to achieve a general conception of matter as having only “geometrical” properties, viz., size, shape, and motion.

There are also differences among interpreters concerning the relative priority in Descartes' philosophical endeavors of epistemology or the theory of knowledge as opposed to metaphysics or first philosophy. In the account of Descartes' development that I have given, he was interested in epistemological and methodological questions first, and these interests came to a head in the Rules . Thereafter, his aim was to establish a new natural philosophy based on a new metaphysics. In the extant works from the 1630s, the World and Discourse plus essays, he argued for the general principles of his physics, including his conception of matter, on empirical grounds. He argued from explanatory scope and theoretical parsimony. As regards parsimony or simplicity, he pointed out that his reconceived matter had only a few basic properties (especially size, shape, and motion), from which he would construct his explanations. He claimed great explanatory scope by contending that his explanations could extend to all natural phenomena, celestial and terrestrial, inorganic and organic. But throughout the 1630s, Descartes claimed that he also was in possession of a metaphysics that could justify the first principles of his physics. His arguments to establish that metaphysics are found in the Meditations and Principles .

Some scholars emphasize the epistemological aspects of Descartes' work, starting with the Rules and continuing through to the Principles . Accordingly, the main change in Descartes' intellectual development is the introduction of skeptical arguments in the Discourse and Meditations . Many interpreters, represented prominently in the latter twentieth century by Richard Popkin (1979), believe that Descartes took the skeptical threat to knowledge quite seriously and sought to overcome it in the Meditations . By contrast, in the main interpretive thread that I follow here, skeptical arguments were a cognitive tool that Descartes used in order to guide the reader of the Meditations into the right cognitive frame of mind for grasping the first truths of metaphysics.

The reader who is curious about these issues should read the relevant works of Descartes, together with his correspondence from the latter half of the 1630s and early 1640s.

3. A New Metaphysics

Descartes first presented his metaphysics in the Meditations , and then reformulated it in textbook-format in the Principles . His metaphysics sought to answer these philosophical questions: How does the human mind acquire knowledge? What is the mark of truth? What is the actual nature of reality? How are our experiences related to our bodies and brains? Is there a benevolent God, and if so, how can we reconcile his existence with the facts of illness, error, and immoral actions?

Descartes had no doubt that human beings know some things and are capable of discovering others, including (at least since his metaphysical insights of 1629) fundamental truths about the basic structure of reality. Yet he also believed that the philosophical methods taught in the schools of his time and used by most of his contemporaries were deeply flawed. He believed that the doctrines of scholastic Aristotelian philosophy contained a basic error about the manner in which fundamental truths, such as the truths of metaphysics, are to be gained. He expressed this mistaken view in the First Meditation, by saying (not in his own voice, but in a voice for the reader): “Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses” (7:18). He then went on to challenge the veridicality of the senses with the skeptical arguments of First Meditation, including arguments from previous errors, the dream argument, and the evil-deceiver argument.

In the Aristotelian scheme against which Descartes is moving, all knowledge arises from the senses, in accordance with the slogan “There is nothing in the intellect that was not previously in the senses” (7:75, 267). Similarly, orthodox scholastic Aristotelians agreed that there is “no thought without a phantasm,” or an image. Descartes explained these convictions as the results of childhood prejudice (7:2, 17, 69, 107; Princ . I.71–3). As children, we are naturally led by our senses in seeking benefits and avoiding bodily harms. As a result, when we grow into adults we are “immersed” in the body and the senses, and so we accept the philosophical view that the senses are the basis for learning about the nature of the reality (7:38, 75, 82–3).

Descartes denied that the senses reveal the natures of substances. He held that in fact the human intellect is able to perceive the nature of reality through a purely intellectual perception. This means that in order to procure the fundamental truths of metaphysics, we must “withdraw the mind from the senses” (7:4, 12, 14) and turn toward our innate ideas of the essences of things, including the essences of mind, matter, and an infinite being (God). Descartes constructed the Meditations so as to secure this process of withdrawal from the senses in Meditation I. Meditation II brings the discovery of an initial truth, in the cogito (7:25), which is elsewhere summarized as the argument “cogito, ergo sum,” or “I think, therefore I am” (7:140). Descartes then observes that the cogito result is known only from the fact that it is “clearly and distinctly” perceived by the intellect (7:35). Hence, he sets up clear and distinct intellectual perception, independent of the senses, as the mark of truth (7:35, 62, 73).

Descartes then unfolds the results of clear and distinct perception in Meditations III–VI, and he repeats and extends these results in Principles I–II. We consider these results in Secs. 3.3–5. For now, let us examine what Descartes thought about the senses as a source of knowledge that was different from the pure intellect.

Descartes famously calls the senses into doubt in the First Meditation, and he affirms in Meditation Six that the senses are not meant to provide knowledge of the “essential nature” of external objects (7:83). In that way, his position in the Meditations differs from that in the Rules , for in that work he allowed that some “simple natures” pertaining to corporeal things can and should be considered through the images of the senses (10:383, 417). In the Meditations , he held that the essence of matter could be apprehended by innate ideas, independently of any sensory image (7:64–5, 72–3). To that extent, his later position agrees with the Platonic tradition in philosophy, which denigrated sensory knowledge and held that the things known by the intellect have a higher reality than the objects of the senses. Descartes, however, was no Platonist, a point to which we will return. His attitude toward the senses in his mature period was not one of total disparagement.

Descartes assigned two roles to the senses in the acquisition of human knowledge. First, he acknowledged that the senses are usually adequate for detecting benefits and harms for the body. Indeed, he considered their natural function to be “to inform the mind of what is beneficial or harmful for the composite of which the mind is a part” (7:83), that is, for the composite of mind and body. In this connection, he was agreeing with the conception of the function of the senses that was widely shared in the traditional literature in natural philosophy, including the Aristotelian literature, as well as in the medical literature on the natural functions of the senses.

Second, he recognized that the senses have an essential role to play in natural philosophy. The older literature on Descartes sometimes had him claiming that he could derive all natural philosophical or scientific knowledge from the pure intellect, independent of the senses. But Descartes knew full well that he could not do that. He distinguished between the general principles of his physics and the more particular mechanisms that he posited to explain natural phenomena, such as magnetism or the properties of oil and water. He claimed to derive the general principles “from certain seeds of truth” that are innate in the mind (6:64). These include the fundamental doctrine that the essence of matter is extension ( Princ . II.3–4, IV.203). As to particular phenomena, in general he had to rely on observations to determine their properties (such as the properties of the magnet), and he acknowledged that multiple hypotheses about subvisible mechanisms could be constructed to account for those phenomena. The natural philosopher must, therefore, test the various hypotheses by their consequences, and consider empirical virtues such as simplicity and scope ( Disc . VI; Princ . IV.201–6). Further, Descartes knew that some problems rely on measurements that can only be made with the senses, including determining the size of the sun (7:80) or the refractive indexes of various materials ( Met . VIII).

In considering Descartes' answer to how we know, we can distinguish classes of knowledge. Metaphysical first principles are known by the intellect acting alone. Such knowledge should attain absolute certainty. Practical knowledge concerning immediate benefits and harms is known by the senses. Such knowledge is usually good enough. Objects of natural science are known by a combination of pure intellect and sensory observation: the pure intellect tells us what properties bodies can have, and we use the senses to determine which particular instances of those properties bodies do have. For submicroscopic particles, we must reason from observed effects to potential cause. In these latter cases, our measurements and our inferences may be subject to error, but we may also hope to arrive at the truth.

At the beginning of the Third Meditation, Descartes declares “I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true” (7:35). Clarity and distinctness of intellectual perception is the mark of truth.

In the fifth set of Objections to the Meditations , Gassendi suggestions that there is difficulty concerning

what possible skill or method will permit us to discover that our understanding is so clear and distinct as to be true and to make it impossible that we should be mistaken. As I objected at the beginning, we are often deceived even though we think we know something as clearly and distinctly as anything can possibly be known. (7:318)

Gassendi has in effect asked how it is that we should recognize clear and distinct perceptions. If clarity and distinctness is the mark of truth, what is the method for recognizing clarity and distinctness?

In reply, Descartes claims that he has already supplied such a method. What could he have in mind? It cannot be the simple belief that one has attained clarity and distinctness, for Descartes himself acknowledges that individuals can be wrong in that belief (7:35, 361). Nonetheless, he does offer a criterion. We have a clear and distinct perception of something if, when we consider it, we cannot doubt it (7:145). That is, in the face of genuine clear and distinct perception, our affirmation of it is so firm that it cannot be shaken.

Descartes held that any act of judgment, such the affirmation “I think, therefore I am,” involves both the intellect and will. The intellect perceives or represents the content of the judgment; the will affirms or denies that content. In the face of genuine clarity and distinctness, “a great light in the intellect” is followed by “a great inclination of the will” (7:59). The inclination of the will is so strong that it amounts to compulsion; we cannot help but so affirm. Descartes thus makes unshakable conviction the criterion. But can't someone be unshakable in their conviction merely because they are stubborn? Assuredly so. But Descartes is talking about the conviction that remains unshakable in face of serious and well-thought out challenges (7:22). To be immune from doubt does mean simply that you do not doubt a proposition, or even that it resists a momentary attempt to doubt; the real criterion for truth is that the content of a proposition is so clearly perceived that the will is drawn to it in such a way that the will's affirmation cannot be shaken even by the systematic and sustained doubts of the Meditations . Perhaps because the process for achieving knowledge of fundamental truths requires sustained, systematic doubt, Descartes indicates that such doubt should be undertaken only once in the course of a life (7:18; 3:695).

Having extracted clarity and distinctness as the criterion of truth at the beginning of the Third Meditation, Descartes immediately calls it into question. He re-introduces an element of the radical doubt from the First Meditation: that a powerful God might have created him with “a nature such that I was deceived even in matters which seemed most evident” (7:36). Descartes therefore launches an investigation of “whether there is a God, and, if there is, whether he can be a deceiver” (7:36).

In the course of the Third Meditation, Descartes constructs an argument for the existence of God that starts from the fact that he has an idea of an infinite being. The argument is intricate. It invokes the metaphysical principle that “there must be at least as much reality in the efficient and total cause as in the effect of that cause” (7:40). This principle is put forward as something that is “manifest by the natural light” (7:40), which itself is described as a cognitive power whose results are indubitable (7:38), like clear and distinct perception (7:144). Descartes then applies that principle not to the mere existence of the idea of God as a state of mind, but to the content of that idea. Descartes characterizes that content as infinite, and he then argues that a content that represents infinity requires an infinite being as its cause. He concludes, therefore, that an infinite being, or God, must exist. He then equates an infinite being with a perfect being, and asks whether a perfect being could be a deceiver. He concludes: “It is clear enough from this that he cannot be a deceiver, since it is manifest by the natural light that all fraud and deception depend on some defect” (7:52).

The second and fourth sets of objections drew attention to a problematic characteristic of this argument. In the words of Arnauld:

I have one further worry, namely how the author avoids reasoning in a circle when he says that we are sure that what we clearly and distinctly perceive is true only because God exists. But we can be sure that God exists only because we clearly and distinctly perceive this. Hence, before we can be sure that God exists, we ought to be able to be sure that whatever we perceive clearly and evidently is true. (7:214).

Arnauld here raises the well-known problem of the Cartesian circle, which has been much discussed by commentators in recent years.

In reply to Arnauld, Descartes claims that he avoided this problem by distinguishing between present clear and distinct perceptions and those that are merely remembered (7:246). He is not here raising a question the reliability of memory (Frankfurt 1962). Rather, his strategy is to suggest that the hypothesis of a deceiving God can only present itself when we are not clearly and distinctly perceiving the infinity and perfection of God, because when we are doing that we cannot help but believe that God is no deceiver. It is as if this very evident perception is then to be balanced with the uncertain opinion that God might be a deceiver (7:144). The evident perception wins out, and the doubt is removed.

Descartes explicitly responds to the charge of circularity in the manner just described. Over the years, scholars have debated whether this response is adequate. Some scholars have constructed other responses on Descartes' behalf, or have found such responses embedded in his text at various locations. One type of response appeals to a distinction between the natural light and clear and distinct perception, and seeks to vindicate the natural light without appeal to God (Jacquette 1996). Another response suggests that, in the end, Descartes was not aiming at metaphysical certainty concerning a mind-independent world, but was merely seeking an internally coherent set of beliefs (Frankfurt 1965). A related response suggests that Descartes was after mere psychological certainty (Loeb 1992). The interested reader can follow up this question by turning to the literature here cited (including Carriero 2008, Doney 1987, and Hatfield 2005).

Building on his claim that clear and distinct perceptions are true, Descartes claims to establish various results concerning the nature of reality, including the existence and fraudlessnes of God, and the natures of mind and matter, to which we turn in the next subsection. Here we must ask: What is the human mind that it can perceive the nature of reality? Descartes has a specific answer to this question: the human mind comes supplied with innate ideas that allow it to perceive the main properties of God (infinity and perfection), the essence of matter, and the essence of mind. For readers in Descartes' day, this claim would naturally raise a further question: assuming that these innate ideas concern “eternal truths” about God, matter, and mind, do these truths hold independent of God, or do they instead reflect the contents of God's own intellect?

Descartes had an interesting answer to this question. He rejected, with many of his contemporaries, the notion that there are eternal truths that obtain independently of the existence of God. Some Neoplatonist philosophers held that the eternal truths in the human mind are copies, or ectypes, of the archetypes in the mind of God. Some Aristotelian philosophers just prior to Descartes, including Francisco Suarez, held that the eternal truths reflect God's own understanding of his creative power; God's power includes that, if he creates a rabbit, it must be an animal. Eternal truths are latent in God's creative power, and he understands this, so that if human beings understand the eternal truths as eternal, they do so by understanding the creative power of God.

Descartes had a different account. He held that the eternal are the free creations of God (1:145, 149, 151; 7:380, 432). God decides what the essence of circle is, or to make 2 + 3 = 5. He might have created other essences, although we are unable to conceive what they might have been. Our conceptual capacity is limited to the innate ideas that God has implanted in us, and these reflect that actual truths that he created. God creates the eternal truths (concerning logic, mathematics, the nature of the good, the essences of mind and matter), and he creates the human mind and provisions it with innate ideas that correspond to those truths.

Descartes reveals his ontology implicitly in the Meditations , more formally in the Replies, and in textbook fashion in the Principles . The main metaphysical results that describe the nature of reality assert the existence of three substances, each characterized by an essence. The first and primary substance is God, whose essence is perfection. In fact, God is the only true substance, that is, the only being that is capable of existing on its own. The other two substances, mind and matter, are created by God and can only exist through his ongoing act of preservation or conservation, called God's “concurrence” ( Princ . I.51).

Descartes' arguments to establish the essences of these substances appeal directly to his clear and distinct perception of those essences. The essence of matter is extension in length, breadth, and depth. One might speak here of “spatial extension,” but with this proviso: that Descartes denied the existence of space separate from matter. Cartesian matter does not fill a distinct spatial container; rather, spatial extension is constituted by extended matter (there is no void, or unfilled space). This extended substance possesses the further “modes” of size, shape, position, and motion. Modes are properties that exist only as modifications of the essential (principal) and the general attributes of a substance. In addition to its essence, extension, matter also has the general attributes of existence and duration. The individual parts of matter have durations as particular modes. All the modes, of matter, including size, shape, position, and motion, can exist only as modifications of extended substance.

The essence of mind is thought. Besides existence and duration, minds have two chief powers or faculties: intellect and will. The intellectual (or perceiving) power is further divided into the modes of pure intellect, imagination, and sense perception. Pure intellect operates independent of the brain or body; imagination and sense perception depend upon the body for their operation. The will is also divided into various modes, including desire, aversion, assertion, denial, and doubt. These always require some intellectual content (whether pure, imagined, or sensory) upon which to operate. Perhaps for that reason, Descartes describes the mind as an “intellectual substance” (7:78; also, 7:12). It seems he held that the mind essentially has a will, but that the intellectual (or perceptive, or representational) power is more basic, because the will depends upon it in its operation.

What role does consciousness play in Descartes' theory of mind? Many scholars believe that, for Descartes, consciousness is the defining property of mind. There is some support for this position in the Second Replies. There Descartes defines mind as “the substance in which thought immediately resides” (7:161). He says of the term “thought” that it extends to “everything that is within us in such a way that we are immediately aware of it” (7:160). If mind is thinking substance and thoughts are essentially conscious, perhaps consciousness is the essence of thought?

Descartes in fact did hold that all thoughts are, in some way, conscious (7:226). He did not mean by this that we have reflective awareness of, and can remember, every thought that we have (5:220). In the Second Meditation, he describes himself as a thinking thing by enumerating all the modes of thoughts of which he is conscious: understanding (or intellection), willing, imagining, and (at this point, at least seeming to have) sense perceptions (7:28). He thus sets up consciousness as a mark of thought. But is it the essence? There is another possibility. If perception (intellection, representation) is the essence of thought, then all thoughts might be conscious in a basic way because the character of the intellectual substance is to represent, and any representation present in an intellectual substance is thereby conscious. Similarly, any act of will present in an intellectual substance also is available to consciousness, because it is of the essence of such a substance to perceive its own states (11:343). Accordingly, perception or representation is the essence of mind, and consciousness follows as a result of the mind's being a representing substance.

In the Discourse , Descartes presented the following argument to establish that mind and body are distinct substances:

Next I examined attentively what I was. I saw that while I could pretend that I had no body and that there was no world and no place for me to be in, I could not for all that pretend that I did not exist. I saw on the contrary that from the mere fact that I thought of doubting the truth of other things, it followed quite evidently and certainly that I existed; whereas if I had merely ceased thinking, even if everything else I had ever imagined had been true, I should have had no reason to believe that I existed. From this I knew I was a substance whose whole essence or nature is simply to think, and which does not require any place, or depend on any material thing, in order to exist. (6:32–3)

This argument moves from the fact that he can doubt the existence of the material world, but cannot doubt the existence of himself as a thinking thing, to the conclusion that his thoughts belong to a nonspatial substance that is distinct from matter.

The problem with this argument is that it is fallacious. It relies on conceivability based in ignorance. For Descartes has not included anything in the argument to ward off the possibility that he, as a thinking thing, is not in fact a complex material system. He has merely relied on the fact that he can doubt the existence of matter to conclude that matter is distinct from mind. But this argument is clearly inconclusive. From the fact that the Joker cannot, at a certain moment, doubt the existence of Batman (because he is with him), but he can doubt the existence of Bruce Wayne (who might, for all the Joker knows, have been killed by the Joker's henchmen), it does not follow that Bruce Wayne is not Batman. In fact, he is Batman. The Joker is merely ignorant of that fact.

In the Meditations , Descartes changed the structure of the argument. In the Second Meditation, he established that he could not doubt the existence of himself as a thinking thing, but that he could doubt the existence of matter. However, he explicitly refused to use this situation to conclude that his mind was distinct from body, on the grounds that he was still ignorant of his nature (7:27). Then, in the Sixth Meditation, having established, to his satisfaction, the mark of truth, he used the mark to frame a positive argument to the effect that the essence of mind is thought, and that a thinking thing is unextended; and that the essence of matter is extension, and that extended things cannot think (7:78). He based this argument on clear and distinct intellectual perceptions of the essences of mind and matter, not on the fact that he could doubt the existence of one or the other.

The conclusion of the Sixth Meditation constitutes the well-known substance dualism of Descartes. That dualism leads to problems. As Princess Elisabeth, among others, asked Descartes: if mind is unextended and matter is extended, how do they interact? This problem vexed not only Descartes, who admitted to Elisabeth that he didn't have a good answer (3:694), but it also vexed Descartes' followers and other metaphysicians. It seems that, somehow, the states of mind and body must be brought into relation, because when we decide to pick up a pencil our arm actually moves, and when light hits our eyes we experience the visible world. But how do mind and body interact? Some of Descartes' followers adopted an occasionalist position, according to which God mediates the causal relations between mind and body; mind does not affect body, and body does not affect mind, but God gives the mind appropriate sensations at the right moment, and he makes the body move by putting it into the correct brain states at a moment that corresponds to the volition to pick up the pencil. Other philosophers adopted yet other solutions, including the monism of Spinoza and the pre-established harmony of Leibniz.

In the Meditations and Principles , Descartes did not focus on the metaphysical question of how mind and body interact. Rather, he discussed the functional role of mind–body union in the economy of life. As it happens, our sensations serve us well in avoiding harms and pursuing benefits. Pain-sensations warn us of bodily damage. Pleasure leads us to approach things that (usually) are good for us. Our sense perceptions are reliable enough that we can distinguish objects that need distinguishing, and we can navigate as we move about. As Descartes saw it, “God or nature” set up these relations for our benefit. They are not perfect. Sometimes we feel pain because a nerve has been damaged, and yet there is no tissue damage at the place in which the pain is felt. Descartes observed that amputees may feel pain in their fingers when they have no fingers ( Princ . IV.196). Sometimes we experience sensory illusions. All the same, he regarded the union of mind and body to have been instituted by God in the best manner possible for finite beings such as ourselves (7:88). Further, we can use our intellects to interpret illusions or other false sensations (7:438).

In discussing the mark of truth, Descartes suggested that the human intellect is as reliable as it is because it was created by God. In discussing the functioning of the senses to preserve or maintain the body, he explained that God has arranged the rules of mind–body interaction in a manner that is conducive to the good of the body. Nonetheless, in each case, errors occur, just as, more broadly, human beings make poor moral choices, even though God has given them a will that is intrinsically drawn to the good (1:366, 5:159, Princ . I.42).

Descartes responded these problems differently. He explained cognitive and moral errors as resulting from human freedom. God provides human beings with a will, and wills are intrinsically free. In this way, there is no difference in degree in freedom between God and man. But human beings have finite intellects. And because they are free, they can choose to judge in cognitive or moral situations for which they do not have clear and distinct perceptions of the true or the good. If human beings restricted their acts of will to cases of clear and distinct perception, they would never err. But the vicissitudes of life may require judgments in less than optical circumstances, or we may decide to judge even though we lack a clear perception. In either case, we may go wrong.

Matters are different for the errors of the senses. The senses depend on media and sense organs, and on nerves that must run from the exterior of the body into the brain. God sets up the mind–body relation so that our sensations are good guides for most circumstances. But the media may be poor (the light may not be good), circumstances may be unusual (as with the partially submerged stick that appears as if bent), or the nerves may be damaged (as with the amputee). In these cases, the reports of the senses are suboptimal. Since God has set up the system of mind–body union, shouldn't God be held accountable for the fact that the senses can mislead? Here Descartes does not appeal to our freedom not to attend to the senses, for in fact we must often use the senses in suboptimal cognitive circumstances when navigating through life. Rather, he points out that God was working with the finite mechanisms of the human mind and body, and he suggests that God did the best he could (7:88).

When Descartes was at La Fleche, there already were signs that the conception of the universe was changing. Recall that Galileo's discovery of four moons of the planet Jupiter was celebrated at La Fleche in 1610. More generally, Copernicus had, in the previous century, offered a forceful argument for believing that the sun, not the earth, is at the center of the solar system. Early in the seventeenth century, Johannes Kepler announced new results in optics, concerning the formation of images, the theory of lenses, and the fact that the retinal image plays a central role in vision. By the early 1630s, Descartes was aware (1:263) of William Harvey's claim that the blood circulates in the body.

Descartes himself contributed some specific new results to the mathematical description of nature, as co-discoverer of the sine law of refraction, and as developer of an accurate model of the rainbow. Nonetheless, as significant as these results are, his primary contribution to the “new science” lay in the way in which he described a general vision of a mechanistic approach to nature and sketched in the details of that vision to provide a comprehensive alternative to the dominant Aristotelian physics.

In the textbooks of Aristotelian physics of Descartes' day, it was common to divide physics into “general” and “special.” General physics pertained to the basic Aristotelian principles for analyzing natural substances: form, matter, privation, cause, place, time, motion. Special physics concerned actually existing natural entities, divided into inanimate and animate. Inanimate physics further divided into celestial and terrestrial, in accordance with the Aristotelian belief that the earth was at the center of the universe, and that the earth was of a different nature than the heavens (including the moon, and everything beyond it). Inanimate terrestrial physics first covered the four elements (earth, air, fire, and water), then the “mixed” bodies composed from them, including the various mineral kinds. Animate terrestrial physics concerned the various powers that Aristotelians ascribed to the ensouled beings, where the soul is considered as a principle of life (possessing vital as well as mental or cognitive powers). In the simplest textbooks, the powers of the soul were divided into three groups: vegetative (including nutrition, growth, and reproduction), which pertained to both plants and animals; sensitive (including external senses, internal senses, appetite, and motion), which pertain to animals alone; and rational powers, pertaining to human beings alone. All the bodies in both inanimate and animate terrestrial physics were governed by a “form” or active principle, as described in Section 1.3.

Descartes' ambition was to provide replacements for all the main parts of Aristotelian physics. In his physics, there is only one matter and it has no active forms. Thus, he dissolved the boundary that had made the celestial and the terrestrial differ in kind. His one matter had only the properties of size, shape, position, and motion. The matter is infinitely divisible and it constitutes space; there is no void, hence no spatial container distinct from matter. The motions of matter are governed by three laws of motion, including a precursor to Newton's law of inertia (but without the notion of vector forces), and a law of impact. Descartes' matter possessed no “force” or active agency; the laws of motion were decreed by God and were sustained by his activity. Earth, air, fire, and water were simply four among many natural kinds, all distinguished simply by the characteristic sizes, shapes, positions, and motions of their parts.

Although Descartes nominally subscribed to the Biblical story of creation, in his natural philosophy he presented the hypothesis that the universe began as a chaotic soup of particles in motion and that everything else was subsequently formed as a result of patterns that developed within this moving matter. Thus, he conceived that many suns formed, around which planets coalesced. On these planets, mountains and seas formed, as did metals, magnets, and atmospheric phenomena such as clouds and rain. The planets themselves are carried around the sun in their orbits by a fluid medium that rotates like a whirlpool or vortex. Objects fall to earth not because of any intrinsic “form” that directs them to the center of the universe, and also not because of a force of attraction or other downward-tending force. Rather, they are driven down by the whirling particles of the surrounding ether. Descartes insisted that all cases of apparent action at a distance, including magnetism, must be explained through the contact of particle on particle. He explained magnetism as the result of cork-screw shaped particles that spew forth from the poles of the earth and flow from north to south, causing magnetized needles to align with their flow.

Descartes also wanted to provide an account of the formation of plants and animals by mechanical causes, but he did not succeed during his lifetime in framing an account that he was willing to publish. He did, however, develop an extensive physiological description of animal bodies, in which he explained the functions of life in purely mechanical manner, without appeal to a soul or vital principle.

In mechanizing the concept of living thing, Descartes did not deny the distinction between animate and inanimate, but he redrew the line between ensouled and unensouled beings. In his view, among earthly beings only humans have souls. He thus equated soul with mind: souls account for intellection and volition, including conscious sensory experiences, conscious experience of images, and consciously experienced memories. Descartes regarded nonhuman animals as machines, devoid of mind and consciousness. This meant that he was required to explain all of the powers that Aristotelians had ascribed to the vegetative and sensitive soul by means of purely material and mechanistic processes (11:202). These mechanistic explanations extended, then, not merely to nutrition, growth, and reproduction (which he wrote about but never achieved results that were published during his lifetime), but also to the functions of the external and internal senses, including the ability of nonhuman animals to respond via their sense organs in a situationally appropriate manner: to approach things that are beneficial to their body (including food), and to avoid danger (as the sheep avoids the wolf).

In the Treatise on Man and Passions , Descartes described purely mechanical processes in the sense organs, brain, and muscles, that were to account for the functions of the sensitive soul. These processes involved “animal spirits,” or subtle matter, as distilled out of the blood at the base of the brain and distributed down the nerves to cause muscle motions in accordance with brain structures and current sensory stimulation. The brain structures that mediate behavior may be innate or acquired. Descartes ascribed some things that animals can do to instinct; other aspects of their behavior he explained through a kind of mechanistic associative memory. He held that human physiology is similar to nonhuman animal physiology, as regards both vegetative and (some) sensitive functions—those sensitive functions that do not involve consciousness or intelligence:

Now a very large number of the motions occurring inside us do not depend in any way on the mind. These include heartbeat, digestion, nutrition, respiration when we are asleep, and also such waking actions as walking, singing, and the like, when these occur without the mind attending to them. When people take a fall, and stick out their hands so as to protect their head, it is not reason that instructs them to do this; it is simply that the sight of the impending fall reaches the brain and sends the animal spirits into the nerves in the manner necessary to produce this movement even without any mental volition, just as it would be produced in a machine. (7:229–30)

Many of the behaviors of human beings are actually carried out without intervention from the mind.

The fact that Descartes offered mechanistic explanations for many features of nature does not mean that his explanations were successful. Indeed, his followers and detractors debated the success of his various proposals for nearly a century after his death. His accounts of magnetism and gravity were challenged. Leibniz challenged the coherence of Descartes' laws of motion and impact. Newton offered his own laws of motion and an inverse square law of gravitational attraction. His account of orbital planetary motions replaced Descartes' vortexes. Others struggled to make Descartes' physiology work. There were also deeper challenges. Some wondered whether Descartes could actually explain how his infinitely divisible matter could coalesce into solid bodies. Why shouldn't collections of particles act like whiffs of smoke, that separate upon contact with large particles? Indeed, how do particles themselves cohere?

Such problems were real, and Descartes' physics was abandoned over the course of the eighteenth century. Nonetheless, it provided a conception for a comprehensive replacement of Aristotelian physics that persisted in the Newtonian vision of a unified physics of the celestial and terrestrial realms, and that continued in the mechanistic vision of life that was revived in the latter part of the nineteenth century.

As the new “mechanical philosophy” of Descartes and others replaced the Aristotelian physics, the theory of sensory qualities had to undergo substantial change. This was especially true for what came to be known as the secondary qualities (in the terminology of Robert Boyle and John Locke). The secondary qualities include colors, sounds, odors, tastes, and tactile qualities such as hot and cold. The Aristotelians maintained that these qualities exist in objects as “real qualities” that are like instances or samples of the quality as experienced. A red thing possesses the quality red in just the same way it possesses a shape: it simply is red, and we experience that very redness when we see a red object.

Descartes sought to replace “real qualities” with a mechanistic account of qualities in objects. He rendered light as a property of particles and their motions: it is a “tendency to move” as found in a continuous medium and radiating out from a luminous body. When light strikes an object, the particles that constitute light alter their rotation about their axis. “Spin” is what makes light have one color rather than another. When particles with one or another degree of spin interact with the nerves of the retina, they cause those nerves to jiggle in a certain way. This jiggling is conveyed to the brain where it affects the animal spirits, which in turn affect the mind, causing the mind to experience one or another color, depending on the degree of spin and how it affects the brain. Color in objects is thus that property of their surface that causes light particles to spin in one way or another, and hence to cause one sort of sensation or another. There is nothing else in the surface of an object, as regards color, than a certain surface-shape that induces one or another spin in particles of light.

Descartes introduced this new theory of sensory qualities in the first six chapters of the World . There, he defended it on the grounds of that his explanation of qualities in bodies in terms of size, shape, and motion are clearly understood, by comparison with the Aristotelian qualities (11:33). Subsequently, in the Meditations and Principles , he defended this account by appeal to the metaphysical result that body possesses only geometrical modes of extension. Real qualities are ruled out because they are not themselves instances of size, shape, or motion (even if patches of color have a size and a shape, and can be moved about).

In addition to a new theory of sensory qualities, Descartes offered theories of the way in which the spatial properties—size, shape, distance, and position—are perceived in vision. Optics had been an area of inquiry since antiquity. Euclid and Ptolemy had each written on optical problems. During the Middle Ages, the Arabic natural philosopher Ibn al-Haytham produced an important new theoretical work in which he offered an extensive account of the perception of spatial properties.

The theoretical terrain in optics changed with Kepler's doctrine that vision is mediated by the retinal image and that the retina is the sensitive body in the eye. Previous theorists generally believed that the “crystalline humor,” now known as the lens, was the sensitive body. Descartes accepted Kepler's result and framed a new theory of spatial perception. Some of his theorizing simply adapted Ibn al-Haytham's theories to the newly discovered retinal image. Thus, Ibn al-Haytham held that size is perceived by combining the visual angle that a body subtends with perception of its distance, to arrive at a perception of the true size of the object. In al-Haytham's scheme, visual angle is registered at the surface of the crystalline humor. Descartes held that size is perceived by combining visual angle with perceived distance, but now he treated visual angle as the extent of an object's projection onto the retina.

In Ibn al-Haytham's account, distance may be inferred if the size of an object is known; then, its visual angle is an inversely proportional to its distance. Descartes recognized this traditional account, depending as it does on past experience of an object's size and on an inference or rapid judgment that combines perceived visual angle with known or remembered size. Ibn al-Haytham also explained that distance can be perceived by an observer's being sensitive to the number of equal portions of ground space that lie between the observer and a distant object. Descartes did not adopt this explanation. However, Descartes used his mechanistic physiology to frame a new account of how distance might be perceived, a theory different from anything that could have been found in Ibn al-Haytham.

In Kepler's new theory of how the eye works, an image is formed on the retina as a result of refraction by the cornea and lens. For objects at different distances, the focal properties of the system must be changed, just as the focal length of a camera is changed. There were several theories of how this might occur, but Descartes accepted the view that the lens changes shape or “accommodates” for near and far vision. He then theorized that this change in the shape of the lens must be controlled by muscles, which themselves are controlled by nerve processes in the brain. He realized that the central nervous state that controls accommodation would vary directly in proportion to the distance of objects. However, unlike the case of inferring distance from known size and visual angle, Descartes did not suppose that the mind is aware of the apparatus for controlling the accommodation of the eye. Rather, he supposed that, by an innate mechanism, the central brain state that varies with distance directly causes an idea of distance in the mind (6:137; 11:183). This physiologically produced idea of distance could then be combined with perceived visual angle in order to perceive an object's size, as in al-Haytham's theory of size perception.

The things that readers find valuable in Descartes' work have changed over the centuries. We have seen that his natural philosophy had an immediate impact that lasted into the eighteenth century. His theory of vision was part of that heritage, as were his results in mathematics. We have also seen that his mechanistic account of the psychology of the sensitive soul and his view that animals are like machines were revived in the nineteenth century.

The fortune of the metaphysical and epistemological aspects of Descartes' philosophy is complex. In his own time, he inspired a raft of followers, who sought to develop his metaphysics, epistemology, natural philosophy, and even to add a worked-out ethics. These authors included Geraud de Cordemoy, Arnold Geulincx, Antoine Le Grand, Nicolas Malebranche, Pierre Regis, and Jacques Rohault. The British philosopher Henry More at first followed Descartes but subsequently turned against him. Other major philosophers, including Benedict de Spinoza and G. W. Leibniz, were influenced by Descartes' thought but developed their own, distinct systems.

Philosophers in subsequent ages have stressed various aspects of Descartes' philosophy, in some cases without undertaking close study of his writings. In the eighteenth century, Hume and Kant (each in his own way) rejected the metaphysical aspirations of Descartes, to know the nature of reality as it is in itself. They did not merely deny his particular metaphysical theories; they rejected his sort of metaphysical project altogether.

During the twentieth century, two aspects of his philosophy became widely invoked and perhaps just as widely misinterpreted. The first is Descartes' skepticism. In the early twentieth century, one response to the threat of skepticism was to retreat to the position that we can only know our own sense data, where “sense data” are equated with the supposed contents of immediate sensory experience: for vision, color patches having a shape. Some authors then treated Descartes' project in the Meditations as that of reducing human knowledge to immediate sense data, from which knowledge of the external world was then to be constructed.

As a reading of Descartes, this position has little to offer. As we have seen, in the Second and Third Meditations Descartes argues from the indubitability of the cogito reasoning to the trustworthiness of intellectual perception to the existence of a perfect being (God). In the latter argument, he does indeed seek to infer the reality of a being external to himself. But the inference has nothing to do with sensory experience. The inference proceeds from a nonsensory and innate idea of God to the existence of that God. Whatever one may think of the quality of the argument, it has nothing to do with sense data. Descartes used skeptical arguments as a tool to disengage the reader from the sensory world in order to undertake metaphysical investigations. There did result, in the Sixth Meditation, a re-evaluation of the senses in relation to metaphysics. But again, sense data were not in the mix.

Another prominent line of twentieth-century interpretation also focuses on the isolation of the subject in the Second Meditation. In the course of that Meditation, Descartes accepts that he knows the contents of his mind, including putative sensory experiences, even though he doubts the existence of his body. Some philosophers have concluded from this that Descartes believed that human beings actually can, in their natural state, have sensory experiences even if they lack a body. But Descartes in fact denied that possibility. In his metaphysics, sense perception and imagination depend for their existence on mind–body union. There can be intellectual perceptions that do not depend on the brain. But acts of imagination and sense perception depend on the brain for their occurrence ( Pass . I.19–20, 43). Thus, Descartes did not in fact hold that we might have all of our sense experiences even if we had no brain. Rather, he allowed that he could conceive his sensory experiences independent of the brain, and that, if God were not supremely good, God could produce those experiences in us independent of the brain. But conceivability does not in all cases—and especially not in cases of mere ignorance, as in the Second Meditation—yield metaphysical possibility (as we have seen in the Discourse argument for the mind–body distinction).

In Descartes' view, it is impossible that a deceiving God would cause our sensory experiences (since God is essentially perfect and so supremely good). Descartes denied that human beings can clearly conceive that God is a deceiver (7:144), and he also maintained that it is “self-contradictory that men [human beings] should be deceived by God” (7:428). But, as he argued in the Sixth Meditation, if our sensory perceptions were not in fact caused by bodies, God would be a deceiver (7:80). Since we can neither clearly conceive of him as a deceiver nor allow the possibility that he is a deceiver, we cannot take seriously the possibility that he produces our sensory experiences in the absence of the existence of material things and, more specifically, in the absence of the relevant brain states.

In the end, Descartes' legacy in part consists of problems he raised, or brought into prominence, but did not solve. The mind–body problem is a case in point. Descartes himself argued from his ability clearly and distinctly to conceive mind and body as distinct beings to the conclusion that they really are separate substances. Most philosophers today accept neither the methodological basis for his claim nor the claim itself. Indeed, since the time of Kant, few philosophers have believed that the clear and distinct thoughts of the human mind are a guide to the absolute reality of things. Hence, the notion that even clear conceivability discerns metaphysical possibility is not accepted. Moreover, few philosophers today are substance dualists.

All the same, the mind–body problem persists. In distinguishing the domain of the mental from that of the physical, Descartes struck a chord. Many philosophers accept the conceptual distinction, but remain uncertain of the underlying metaphysics: whether mind is identical with brain; or the mental emerges from complex processes in the brain; or constitutes a property that is different from any purely physical property, even while being instantiated by the brain. In this case, a problem that Descartes made prominent has lived far beyond his proposed solution.

Bibliography

NOTE: In referring to Descartes' works, I have used the pagination of the Adam and Tannery volumes (AT), Oeuvres de Descartes , 11 vols. The citations give volume and page numbers only (dropping the abbreviation “AT”). Whenever possible, I have used the Cottingham, Stoothoff, Murdoch, and Kenny translation, The Philosophical Writings Of Descartes , 3 vols., which shows the AT pagination in the margins. The AT volume numbers provide a guide to which work is being cited in translation: vols. 1–5, correspondence; vol. 6, Discourse ; vol. 7, Meditations ; vol. 10, Rules ; vol. 11:1–118, World, or Treatise on Light ; vol. 11:119–222, Treatise on Man ; vol. 11:301–488, Passions . Where there is no accessible translation for a citation from AT, I show the citation in italics . I also cite works that are broken into articles by abbreviated title, part, and article: Princ . for the Principles , and Pass . for the Passions .

• Carriero, John. 2008. “Cartesian Circle and the Foundations of Knowledge,” in Companion to Descartes , ed. Janet Broughton and John Carriero. Malden, Mass.: Blackwell.
• Descartes, René 1964–76. Oeuvres de Descartes , 11 vols., ed. Charles Adam and Paul Tannery. Paris: Vrin/CNRS. Cited by volume and page number.
• Doney, Willis, ed., 1987. Eternal Truth and the Cartesian Circle . New York: Garland Publishing.
• Frankfurt, Harry G. 1962. “Memory and the Cartesian Circle,” Philosophical Review , 71: 504–11.
• Frankfurt, Harry G. 1965. “Descartes' Validation of Reason,” American Philosophical Quarterly , 2: 149–56.
• Garber, Daniel. 1992. Descartes' Metaphysical Physics . Chicago: University of Chicago Press.
• Hatfield, Gary. 2005. “Cartesian Circle,” in Blackwell Guide to Descartes' Meditations , ed. S. Gaukroger. Oxford: Blackwell, 122–41.
• Jacquette, Dale. 1996. “Descartes' Lumen Naturale and the Cartesian Circle,” Philosophy and Theology: Marquette University Quarterly , 9: 273–320.
• Loeb, Louis. 1992. “Cartesian Circle,” in Cambridge Companion to Descartes , ed. J. Cottingham. Cambridge: Cambridge University Press, 200–35.
• Machamer, Peter, and J. E. McGuire. 2006. “Descartes's Changing Mind,” Studies In History and Philosophy of Science , 37: 398–419.
• Popkin, Richard H. 1979. History of Scepticism from Erasmus to Spinoza . Berkeley: University of California Press.
• Schuster, John. 1980. “Descartes' Mathesis Universalis , 1619–28,” in Descartes: Philosophy, Mathematics and Physics , ed. S. Gaukroger. New Jersey: Barnes and Noble, 41–96.
• Sebba, Gregor. 1987. Dream of Descartes . Carbondale, Ill.: Southern Illinois University Press.
• Watson, Richard. 2007. Cogito, Ergo Sum , rev. edn. Boston: Godine.
• Descartes, René. 1965. Discourse on Method, Optics, Geometry, and Meteorology , trans. Paul J. Olscamp. Indianapolis: Bobbs-Merrill. Originally published in French in 1637.
• –––. 1983. Principles of Philosophy , trans. V. R. Miller and R.P. Miller. Dordrecht: Reidel. Originally published in Latin in 1644.
• –––. Philosophical Writings of Descartes , 3 vols., trans. John Cottingham, Robert Stoothoff, Dugald Murdoch, and Anthony Kenny (Cambridge: Cambridge University Press, 1984–91).
• –––. 1989. Passions of the Soul , trans. Stephen H. Voss Indianapolis: Hackett. Originally published in French in 1649.
• –––. 1998. World and Other Writings , trans. Stephen Gaukroger. Cambridge: Cambridge University Press.
• Broughton, Janet, and John Carriero, eds. 2008. Companion to Descartes . Malden, Mass.: Blackwell.
• Clarke, Desmond M. 1982. Descartes' Philosophy of Science . University Park: Pennsylvania State University Press.
• Cottingham, John, ed. 1992. Cambridge Companion to Descartes . Cambridge: Cambridge University Press.
• –––, ed. 1994. Reason, Will and Sensation: Studies in Descartes's Metaphysics . Oxford: Clarendon Press.
• –––, ed. 1998. Descartes . Oxford: Oxford University Press.
• Curley, Edwin. 1978. Descartes against the Skeptics . Cambridge, Mass.: Harvard University Press.
• Dicker, Georges. 1993. Descartes: An Analytical and Historical Introduction . New York: Oxford University Press.
• Frankfurt, Harry. 1970. Demons, Dreamers, and Madmen . Indianapolis: Bobbs-Merrill.
• Gaukroger, Stephen. 1995. Descartes: An Intellectual Biography . Oxford: Oxford University Press.
• –––. 2002. Descartes' System of Natural Philosophy . Cambridge: Cambridge University Press.
• Gaukroger, Stephen, John Schuster, and John Sutton, eds. 2000. Descartes' Natural Philosophy . London: Routledge.
• Guèroult, Martial. 1984–85. Descartes' Philosophy Interpreted According to the Order of Reasons , trans. R. Ariew, 2 vols. Minneapolis: University of Minnesota Press.
• Hatfield, Gary. 2003. Descartes and the Meditations . London: Routledge.
• Nelson, Alan, ed. 2005. Blackwell Companion to Rationalism . Oxford: Blackwell.
• Rodis-Lewis, Geneviève. 1998. Descartes: His Life and Thought , trans. J. M. Todd. Ithaca: Cornell University Press.
• Rorty, Amélie, ed. 1986. Essays on Descartes' Meditations . Berkeley: University of California Press.
• Shea, William R. 1991. Magic of Numbers and Motion . Canton, Mass.: Science History Publications.
• Smith, Norman Kemp. 1953. New studies in the Philosophy of Descartes: Descartes as Pioneer . London: Macmillan.
• Voss, Stephen, ed. 1993. Essays on the Philosophy and Science of René Descartes . New York: Oxford University Press.
• Williams, Bernard. 1978. Descartes, The Project of Pure Inquiry . London: Penguin.
• Wilson, Catherine. 2003. Descartes's Meditations: An Introduction . Cambridge: Cambridge University Press.
• Wilson, Margaret D. 1978. Descartes . London: Routledge & Kegan Paul.
• Descartes , resources from PhilWeb

Arnauld, Antoine | Cordemoy, Geraud de | Descartes, René: and the pineal gland | Descartes, René: epistemology | Descartes, René: ethics | Descartes, René: life and works | Descartes, René: modal metaphysics | Descartes, René: ontological argument | Descartes, René: physics | Descartes, René: theory of ideas | emotion: 17th and 18th century theories of | Gassendi, Pierre | Le Grand, Antoine | More, Henry | Regius, Henricus

Acknowledgments

The editors would like to thank Gintautas Miliauskas (Vilnius University) for notifying us of a variety of typographical errors in this entry.

René Descartes ( 1596-1650 )

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Descartes: A Biography

Desmond M. Clarke, Descartes: A Biography , Cambridge University Press, 2006, 520pp., $40.00 (hbk), ISBN 0521823013.

Reviewed by Karen Detlefsen, University of Pennsylvania

There has developed, in recent years, a small industry in the writing of philosophical biographies -- books detailing the lives of great philosophers, providing background to the development of their thoughts, or describing the relationships between philosophers. Descartes scholarship is no exception, and Desmond Clarke's recently published biography joins other titles on Descartes published in the last decade or so by Stephen Gaukroger, Geneviève Rodis-Lewis, and Richard Watson. [1] Clarke's work is notable for its exhaustive detail, drawing helpfully upon Descartes' voluminous and revealing correspondence to reconstruct as best as possible Descartes' movements and mindsets throughout his almost 54 years of life. His focus is on Descartes' life, but he also provides succinct and accessible descriptions of the major developments in Descartes' philosophical thinking. Another virtue of the book is the context -- theological, political, social, and scientific -- that Clarke provides for the reader to better understand Descartes as a person and as an intellectual. This biography will appeal to the scholar of Descartes and the seventeenth century, as well as to other philosophers interested in the lives of the greatest thinkers in their profession's history. It is also appropriate for a lay reader keen on gaining insight into Descartes' contribution to philosophy and why he is considered to be such a pivotal figure in the history of the discipline.

Three related themes edge repeatedly to the forefront throughout the book. They are: first, Descartes' seemingly endless travels across the continent and his eventual isolation in voluntary exile; second, Descartes' own largely unflattering character; and third, the ubiquitous and sometimes menacing presence of religious authorities exercising an influence over Descartes' life and work, especially his scientific work.

The theme of travel emerges early in Clarke's book, and it persists to the end: "The fate of Descartes' remains reflects his life-long penchant for changing residence … " (412). While living, Descartes first left France in order to avoid one of the obvious careers for which his education had groomed him -- that of teacher, lawyer, or joining the clergy -- and he traveled for ten years (32-3). While he did continue his research during this time (33), one gets the sense that his journeys were unstructured and drifting (37, 93). Beeckman, one of his earliest close friends, "remarked, on one occasion, that Descartes suffered from wanderlust [ peregrinandi cupidus ]" (94). Descartes' portrayal, in the Discourse on Method , of his ten years of travel as quite purposeful seems, then, to have been a retrospective evaluation (94). Nonetheless, the time that Descartes spent out of France had an enormous impact on his thought and works. He saw travel as a replacement for study in schools and through books, of which he read few (35, 107, 177). Indeed, the fact that he owned very few books throughout his life certainly made it easier to move at the frantic pace he kept up (109). His aversion to the ideas of others extended to his avoidance of learned people (68). In fact, as he matured, he tended to avoid all contact with people, and his adult life was lived primarily in isolation.

Descartes'… frequent changes of address, and his almost obsessive efforts to conceal his precise location even from trusted friends, make [it plausible that his second departure from France was to avoid the customary distractions of a gentleman's life (such as visiting friends and attending court)]. (97)

So elusive was he that some spelled his name "Monsieur d'Escartes" (Mr. Evasion) (97). This frequent movement and his resulting isolation contributes to Clarke's characterization of Descartes as a lonely, yet somewhat unpleasant, figure: "He had become [by 1638] a reclusive, cantankerous, and oversensitive loner, who worried incessantly about his place in history and the priority he claimed for various discoveries" (180; see also 214, 249). It is true that Descartes slowed his pace of travel as he grew older; the winter of 1645-46 seems to have been a significant turning point in Descartes' life.

In contrast with the confident expectations of a long, healthy life and with the combative confrontations with almost all his correspondents that characterized the early 1640s, Descartes acknowledged a growing awareness of advancing years and a willingness to resile from controversy in 1645-46. Even his lifelong penchant for traveling seems to have abated … (317)

Still, even with the slowed pace, Descartes made two arduous (for one feeling the effects of his age as Descartes was [317, 337]) trips back to France (1647 and 1648) along with his final journey to Sweden in 1649. As one example of the many helpful extra-textual additions to the book, Clarke provides the reader with an appendix, complete with map, detailing Descartes' movements and residences throughout his life (Appendix 2, 421-23).

Descartes does not come across as an agreeable character in Clarke's biography. We are prepared for this by Clarke's introduction where he tells us of Descartes' "sensitivity to criticism and the certainty that he claimed, prematurely, for his own view", and draws our attention to the fact that Descartes "fought with almost everyone he encountered while constantly announcing that all he wanted was 'the security and tranquility' required to complete his intellectual projects" (5). Among his flaws, we learn of his lack of modesty (81), his paranoia and suspicion (for example, in believing that his private correspondence is being read before reaching its addressee (114)), his reluctance to concede intellectual points (159), his tendency to bear grudges (159), his duplicity (e.g. 197, 223, 382), and his manipulative treatment of people, even of supportive friends (231). It is no wonder, then, that "from Descartes' earliest days in the United Provinces, [it is clear] that he had a weakness for losing friends and, in the case of those who criticized him, for making permanent enemies" (307). While he acquired many acquaintances and some friends over the years, he could not seem to keep them on his side, and the blame seems to lay mostly with Descartes himself. (There were some exceptions, such as Henri Reneri [105] and Cornelis van Hogelande [307] who remained Descartes' friends for life.) The case of Isaac Beeckman provides an example of a once-dear friend who incurs Descartes' fury, in Beeckman's case, for sharing details of Descartes' Compendium of Music with Marin Mersenne (46-52). Of this rift, Clarke writes:

When one finds that Descartes disputed publicly with Fermat, Roberval, Voetius, Bourdin, and many others, the emotionally charged rift with Beeckman assumes the status of the first example of what subsequently emerged as a pattern in his personal and professional life. (52)

The case of Gassendi provides an example of a critic who incurs Descartes' fury due to the fact of his criticism. As a result of their "sharp" exchange, namely the Fifth Objections and Replies to the Meditations , Descartes punished Gassendi by refusing to allow him to see Descartes' replies to the objections and then printing the objections under Gassendi's name, in contrast with the other (anonymously published) objections (205). That occurred in the printing of the first edition of the Meditations . Later, and in response to Gassendi's critical A Metaphysical Disquisition: Or Doubts and Counter Objections to the Metaphysics of René Descartes, and his Replies , Descartes decided "to retaliate… by deleting the Fifth Objections from the next edition of the text… " (279). Descartes seems to have been in almost constant battle with one or another critic or erstwhile friend -- with Voetius (218-45), the Leiden theologians (chapter 12), French mathematicians (168-175), Regius (chapters 8 and 11), and the list goes on. Given this, and given the fact that Descartes himself is portrayed as the principal source of the disputes, the reader wonders if Descartes was completely disingenuous or rather self-deceived in his

self-serving description of his attitude to criticism. Not only is he 'docile' [according to his own claim], but he is reluctant to speak in his own defense… . He even claims not to care whether others have anticipated his discoveries, thereby implying that he would not get involved in priority disputes… . The reality was rather different. (158-59; see also 197)

Perhaps the least attractive of his many failings was Descartes' duplicity, on show repeatedly throughout the book. As one example, we see him sending pairs of letters to Queen Christina and to Chanut presenting sharply divergent attitudes toward the Queen's invitation to Sweden. "These parallel letters… ", writes Clarke, "show Descartes at his dissembling best" (384).

While Descartes is portrayed with many flaws and few positive personality traits, the reader gains insight into one reason why Descartes might have been so. He lived under an almost constant threat from various religious authorities, a threat that continuously undermined his ability to write and publish freely, especially on scientific matters. One of the prime virtues of Clarke's book is the extensive context he provides so as to make sense of Descartes' thought. This includes the political and social context, for example, the organization of French society in Descartes' youth (10ff), details on the printing of books in the seventeenth century (137ff), and the political atmosphere in Paris during Descartes' final trip to France (370ff). Of central importance to understanding Descartes, however, is the religious and scientific context that Clarke provides.

Descartes inherited from Copernicus and Galileo the intellectual conflicts involved in attempting to develop the new astronomy and, at the same time, to remain within the Catholic Church. He avoided church censure of his astronomy for almost two decades by dissimulation, self-censorship, and astuteness. However, his ambiguous support for Copernicus was merely a symptom of a much more radical problem that could not be camouflaged as easily. Descartes challenged the fundamental philosophy in terms of which both Catholic and Reformed theologians had expressed their teaching of Christian dogmas for centuries. That could not be marginalized as a technical question in astronomy that only experts might be expected to understand (4).

This background helps the reader understand both Descartes' personality and, to an extent, his hectic movements. Many of Descartes' "battles", in which his cantankerous personality is on display, were centered around potential theological clashes and his attempts to avoid them. Moreover, Descartes undertook at least some of his travels -- for example, his 1619 trip to Germany (52-54) -- in order to find a more open-minded intellectual atmosphere. So it is this third theme -- the religious threats that hovered over Descartes due to his innovations in natural philosophy -- which emerges as fundamental in understanding much about Descartes' life. This may not be news to the Descartes scholar, but Clarke's biography gathers together an extraordinary degree of detail on the theological pressures that Descartes faced, underscoring the shaping force that they had upon Descartes' thought and life. This theme also helps to explain to the non-expert Descartes' towering position in the history of philosophy due to his being at the forefront of the philosophical revolution that was bound up with the scientific revolution of the seventeenth century -- revolutions that forever changed the relations among science, theology, and philosophy.

In addition to these three central themes, Clarke provides extremely clear and crisp expositions of the philosophy found in Descartes' major works -- and also some less well-known pieces such as the essay on love written in response to questions posed by Chanut (338-41). While these are useful reminders for Descartes scholars, the non-expert will find these pages particularly helpful for gaining an insight into Descartes' contributions to the history of philosophy. These sections include an admirably accessible treatment of the scientific essays accompanying the Discourse on Method -- the Dioptrics , Meteors , and Geometry -- works which have been too rarely read alongside the more popular Discourse (144-52). The excellent treatment of the many themes of the Elizabeth correspondence is also worthy of note (254-76). In addition to this exposition, the reader gains a clear view of the general arc and development of Descartes' thought. Broadly, his early interests included mathematics, physics, and astronomy; he undertook a major shift in focus around 1640 when he returned in earnest to metaphysical problems (184); and his interests in physiology and human passions tended to occupy him in his later years. In addition, the non-expert will get a good sense of the fluidity of intellectual boundaries (between metaphysics, natural philosophy or science, theology and so forth) in the seventeenth century, essential for understanding the philosophy of that time. The book is also peppered with wonderful details such as the opportunity Descartes had in the unusually cold winter of 1634-35 to study closely the shapes of snowflakes (130).

Clarke's biography is exhaustively researched and documented. Where details of Descartes' life are sketchy and uncertain, Clarke has done a fair bit of sleuthing around for whatever information is available, and then suggests plausible reconstructions given the limited material. A casual reader of seventeenth-century philosophy or intellectual history who wishes to gain a very general understanding of Descartes' importance may find the book too detailed, but for such a reader, it should be easy enough to focus on passages of interest, skimming over the rest. For the scholar, much of what Clarke writes about Descartes' life, work, and thought was known already, but much is also enriched and expanded, and the scholar will find it an invaluable resource on Descartes' life given the meticulous detail gathered together in one place.