An Introduction to René Descartes

Austin Tannenbaum
7 min readFeb 20, 2021


René Descartes was a French philosopher, scientist, and mathematician of the 17th century. During his lifetime, he participated in a variety of fields, notably optics, geometry, and astronomy. He was the first to publish a text on the laws of refraction, or what governs the bending of light as it passes from one medium to another. He invented analytic geometry, which uses a coordinate system to represent geometric figures with algebraic equations. And he theorized that the sun and planets had coalesced from whirlpools of swirling particles — a precursor to modern astronomy’s nebular hypothesis. He even correctly calculated the angular radius of rainbows formed by the viewing eye and a rainbow’s edge. But while Descartes made many lasting contributions to these and other disciplines, he is best remembered for his proto-Enlightenment philosophy, championing reason rather than received wisdom as the touchstone of truth.

Descartes was born on March 31, 1596 in a northwestern French town called La Haye that has since been renamed “Descartes” in his honor. Descartes’ mother died when he was just 13 months old. Descartes’ father, being a member of the Parliament of Brittnay at Rennes, was unable to care for Descartes, who was raised instead by his grandmother. Descartes himself was a sickly child and was not expected to survive. When he entered a Jesuit secondary school in 1607, he was permitted to sleep in until 10:00 a.m. mass while his classmates were forced to rise every morning at 5:00 a.m. for prayer. Descartes received a rich education in the maths, sciences, and humanities, being exposed to thinkers such as Galileo, Cicero, and Aristotle.

After graduating in 1614, Descartes proceeded to the University of Poitiers, where he spent his next two years. After earning a baccalaureate and a licentiate in civil and canon law in 1616, Descartes “abandoned scholarship” in favor of travel, “resolving to seek no knowledge except what [he] could find in [him]self or read in the great book of the world.” His adventures landed him in military service, first with the Dutch in 1618 and then with the Germans in 1619. It was during this time that Descartes met perhaps his biggest influence, Isaac Beeckman, who introduced him to the idea that mathematical techniques could be applied to other fields. In November 1619, after shutting himself into a room with a charcoal stove to keep warm, Descartes had a succession of visions that convinced him that all truths were interlinked, inspiring him to apply the mathematical method to philosophy.

The next year, Descartes left the army and eventually settled in Paris. During this time, he inherited property, which he sold to buy bonds that financially sustained him for the remainder of his life. In 1628, he relocated again to the Netherlands to focus on writing without “unwelcome visitors…wasting his free time” given the country’s custom of respecting privacy. While in Amsterdam, Descartes became romantically involved with a maid who worked at his lodgings. The two had a daughter named Francine, who was born in 1635 but passed away five years later from scarlet fever.

Having begun to publish works on mathematics, science, and philosophy, Descartes became a prominent intellectual figure in Europe. But controversy accompanied his fame. After being charged with unorthodoxy and threatened with deportation and the public burning of his books, Descartes fled to the Hague and convinced the Duke of Orange to intervene on his behalf.

In the latter years of his life, Descartes wrote on psychology, emotions, and morality. His newfound interest in these subjects was inspired by correspondences with Princess Elisabeth of Bohemia, to whom he dedicated a number of his mature writings. In 1649, he began another royal correspondence, this time with Queen Christina of Sweden. The Queen invited Descartes to Sweden to act as her tutor, which Descartes reluctantly accepted. There, he was made to rise at 5:00 a.m. each morning, contrary to his habit of sleeping in. Less than six months after his arrival, Descartes developed pneumonia and passed away on February 11, 1650.

As a proto-Enlightenment figure, Descartes grounds his philosophy in a thoroughgoing skepticism known as “hyperbolic doubt.” In his intellectual autobiography Discourse on Method, Descartes remarks that although the brightest minds have been working on philosophical questions for millennia, there is no more agreement about the answers than when the discipline first began. Descartes thus resolves to cast aside what has been said in the past and chart his own path toward truth using an investigatory method consisting of four rules:

  1. Only accept what is incapable of being doubted.
  2. Break down large questions into their component parts.
  3. Begin by investigating simple questions and move stepwise to more complex questions.
  4. Ensure that your investigation is comprehensive to avoid overlooking an important piece of information.

The first rule compels Descartes to subject everything — even our most cherished and seemingly self-evident notions — to doubt. In doing so, Descartes discovers that our senses cannot, as commonly believed, be trusted to inform us about the world, as they are liable to myriad misperceptions. Examples given include mistaking a faraway object as small when it is in fact large and mistaking a dream for reality. Furthermore, Descartes finds it plausible that an evil demon or deceiver God could be tricking us into apprehending appearances that do not reflect the true nature of reality.

Having at least temporarily renounced the senses as a reliable source of knowledge, Descartes turns to the mind to see if it is capable of deriving any indubitable truths. In a clever move, Descartes reasons that because doubt is occurring, there must be a doubter performing the action. Although he tries to doubt this, he cannot deny that doubt is present and that he is aware of this doubt. Doubt is a mode of thinking, or mental activity, which leads Descartes to his famous maxim, cogito, ergo sum, or “I think, therefore I am.” Put another way, Descartes reasons that since doubts are being generated and perceived, he must exist as some kind of thinking/doubting thing that is doing the generating and perceiving.

From here, Descartes moves to prove God’s existence and benevolence. He reasons that since he thinks, and since he can think of concepts that he himself does not possess — perfection, infinity, eternality, omniscience — there must be a being who imparted them to him as concepts. Descartes takes as a given that it is impossible for something less perfect to produce something more perfect. Therefore, only a being who actually possesses these traits could create and instill them as concepts.

Importantly, God, being a perfect being, is understood by Descartes to be incapable of imperfections. Deception is an imperfection, because it entails false knowledge. Therefore, God cannot be a deceiver, and must render the world honestly. With this move, Descartes recovers the legitimacy of empirical knowledge, which now may be used, albeit carefully, to aid our understanding of the world.

Interestingly, Descartes, who begins his philosophy with radical skepticism, ends it with metaphysical realism. He points out that sensory perceptions often occur involuntarily, for instance when a gust of wind is tactilely felt on the skin. According to Descartes, stimuli thus exist externally to the senses, implying the presence of a real, mind-independent world.

The last major topic in Descartes’ philosophical system is his philosophy of mind, known as Cartesian dualism. By mind, Descartes does not mean the physical brain, but the experience of self-awareness commonly referred to as consciousness. Descartes begins with a trifecta of premises:

  1. The essence of mind is thought.
  2. The essence of body is extension.
  3. The essence of God is existence.

He then draws attention to the fact that while we can imagine ourselves conscious without a body, we cannot imagine ourselves conscious without thought. Descartes uses this principle to argue that the mind must be a different substance than, and exist separately from the body. In Descartes’ schema, there exist three distinct substances: mind, body, and God. The latter is the ultimate substance responsible for bringing the other two into being.

Now, while Descartes maintains that body and mind are ultimately separate, he acknowledges that during human life, they are inextricably linked: the stimuli received by the body are translated into sensations by the mind, with the pineal gland acting as mediator. His proposed mechanism of action involves “animal spirits,” or “a certain very fine wind or air” that circulates between the body and the brain, helping trigger perceptions along with memories and bodily movements. Descartes also theorizes that animal spirits induce our “passions” or emotions. He identified six basic passions upon which all others are based: wonder, love, hatred, desire, joy, and sadness. Notably, while the biology of Descartes’ own time, let alone of the modern era, invalidated the technical details of these ideas, they remain historically significant for hypothesizing a seat of consciousness and foreshadowing the endocrine system, which releases mood- and behavior-altering hormones.

Descartes’ ultimate purpose in philosophizing was to improve himself, however modestly, regarding the self as the one aspect of life over which one possesses real control. Viewing ethics as a science, Descartes believed that virtue is achieved through correct reasoning, which leads one to choiceworthy actions. In Aristotelian fashion, Descartes maintained that in addition to being choiceworthy, virtue was pleasurable, if in a higher, more spiritual sense than bodily pleasure. Finally, although there is no guarantee that one’s discoveries will produce societal improvement, Descartes held that one has a duty to share them if they could potentially benefit humankind. Despite his wariness about attempting to change a world that is often beyond any one individual’s control, Descartes affirmed the value of collaboration and teamwork for mutual betterment, advocating that,

“The best minds [ought] to try to make further progress by helping with the necessary experiments, each according to his preference and his ability, and by communicating to the public everything they learn. In this way, some could take over where others had left off; and thus, by combining the lives and labours of many people, we might get much further working together than anyone could do on his own.”