JMS Pearce
Hull, England

René Descartes (1596–1650) was a French philosopher, mathematician, and scientist. So profound and diverse were his writings^1,2 that this is no more than a slight sketch of his extraordinarily original ideas and his contributions to medicine.

A year after his birth in Touraine, his mother died in childbirth and his grandmother cared for him. He attended the Jesuit Collège at La Flèche studying mathematics and physics and then moved to the University of Poitiers to graduate in law in 1616. However, traditional academic paths did not satisfy his curious, radically analytical mind. He preferred to learn from life itself, traveling widely “to visit courts and armies, and people of diverse temperaments and ranks to reflect upon whatever came my way to derive some profit from it.”³

In 1618 he served in the army of the Dutch Protestant States. A biographer reported that one night, he had a series of strange dreams that inspired the idea of relating mathematics and geometry to a new philosophy. While in Bohemia in 1619, without formal training, this ingenious man invented analytic geometry using algebraic symbols. He traveled widely and eventually settled in in the Netherlands, where he wrote his major works, revolutionizing much of conventional academic thought.

A devotee of Aristotle, Descartes is often known as the father of modern philosophy, but his contributions to medicine—though less frequently discussed—were both original and influential. Applying his rational philosophy, physiology, and experimental science, he sought to reform medical understanding based on a mechanistic framework and reasoning. In a letter to Huygens in January 1638 he wrote: “Je travaille maintenant a composer un Abrégé de médecin, que je tire en partie des livres et en partie de mes raisonnements.” (“I am now working on composing a compendium of medicine, which I draw partly from books and partly from my own reasoning.”)

Truth by reasoning

He found unreliable the traditional empirical knowledge obtained through the senses or experience, and he developed the idea of doubting everything, including himself. He sought for truth by rational inquiry:

I shall apply myself seriously and freely to the general destruction of all my former opinions… the slightest ground for doubt that I find in any, will suffice for me to reject them.

Meditations on First Philosophy

Descartes insisted that only if an idea that presented itself was both “clear” and “distinct” to rational intuition could it be trusted as true.

He also believed in innate knowledge—that all humans were born with knowledge through the higher power of God, a theory opposed by the empiricist John Locke (1632–1704), who thought all knowledge was acquired through experience.

Cogito, ergo sum

Descartes tried to establish and define existence and reality. He famously coined the statement “Cogito, ergo sum”: “I think, therefore I am”; the fact that you can think and doubt your own existence proves that you do exist.

Cartesian dualism

He argued that mind and body were distinct “principal attributes.” The body was physical and subject to mechanical laws (res extensa), while the mind was immaterial but capable of thought (res cogitans). This dualistic view became a cornerstone of Western thought. However, it posed a problem.⁴ Descartes asked: How, if thoughts were not physical, could they interact with and move the physical body?

Pineal, the seat of the soul

He therefore suggested they were connected in the pineal gland, which he said was the principal seat of the soul, and the place in which all our thoughts are formed. Animal spirits* flowed from the soul in the pineal to determine sensation, memory, and bodily movements. It was the seat of the sensus communis, the general faculty of sense. He used the word “idea” to describe the flow of animal spirits from the pineal gland, which he discussed in The Passions of the Soul (1649), the last book that he published, and in the unfinished Treatise on Man, published posthumously.⁶ Centuries later, the pineal was found to be an endocrine structure which produces the hormone melatonin in response to light and circadian rhythms.

Mechanistic physiology

The metaphysics of Descartes was rationalist, based on innate ideas of mind, matter, and God, but his physics and physiology, based on experiment, were mechanistic and empirical. This approach laid the groundwork for physiology.⁵ Descartes likened the nervous system to a network of tubes through which “animal spirits” flow to animate muscles and organs.

The Traité de l’homme written in the 1630s was published posthumously in its original French in 1664, after a Latin translation (De Homine, 1662 and 1664).⁶ It was planned as an explanation of “all natural phenomena, that is, all of physics.” He proposed that the human body operated like a complex machine, governed by physical laws rather than by spiritual or humoral forces.⁷ He described for the first time involuntary movements triggered by external stimuli. When the body encountered a stimulus—such as touching a hot object—nerves transmitted this information to the brain, which then sent a response to the muscles causing movement. Thus an automatic reaction of the body does not require conscious thought. This idea long predated the modern understanding of the reflex arc.

He also wondered about the actions of the heart and blood, and in 1643 he praised William Harvey’s De Motu Cordis (1628) and supported his theory of the blood’s circulation:

There are many small passages at the extremities of the arteries, through which the blood received by them from the heart passes into the small branches of the veins, whence it again returns to the heart; so that its course amounts precisely to a perpetual circulation…

He further reasoned:

And why should the left cavity of the heart and the great artery be wider and larger than the right cavity and the arterial vein, were it not that the blood of the venous artery, having only been in the lungs after it has passed through the heart, is thinner, and rarefies more readily, and in a higher degree, than the blood which proceeds immediately from the hollow vein.³

Emotions and the passions

In Les passions de l’ame (1649), Descartes identified six primary emotions of the spirit and soul: wonder, love, hatred, desire, joy, and sadness. He challenged the view that the soul was divine; this provoked hostility from the Church despite his sincere belief in God. He suggested emotions were responses to the movements of the animal spirits within the body, influencing both physical and mental states: an idea that anticipated neurotransmitters. This early exploration of psychosomatic relationships led to later studies in physiology and psychiatry. Interestingly, he believed animals had neither a soul nor intelligence, and could not reason or experience suffering, a notion disproved by Darwin’s observations on animals. Descartes’ scientific curiosity extended far beyond philosophy and physiology, notably into the physics of light and vision.

Optics

He independently formulated and described mathematically the law of refraction, previously discovered by Willebrord Snell. Descartes’ La Dioptrique (1637), an appendix to his Discourse on the Method, explained light as a mechanical disturbance. Johannes Kepler (1571–1630) had shown how the eye produced an image on the retina; Descartes then correctly described its projection through the optic nerve to the brain, where it was interpreted. And, he explained how light is refracted and internally reflected within water droplets to produce the rainbow.

He also gave an outline of embryology in L’ Homme de René Descartes, Et La Formation Du Foetus; Ou Traite de La Lumiere, published in 1664.

Last years

In 1649, Queen Christina of Sweden (1626–1689) invited him to visit her in Sweden. She was an eminent scholar, learned in philosophy, mathematics, and alchemy. Descartes gave Queen Christina a few lessons in philosophy and remained in Sweden living with his friend, the French ambassador Pierre Chanut. On February 1, 1650, Chanut developed pneumonia from which he recovered. Descartes at the same time became ill and died on February 11, 1650.⁸ His death was also attributed to pneumonia. However, letters from Dr. Jan van Wullen, a Dutch physician in Stockholm summoned by Queen Christina to treat Descartes, allegedly described skin pigmentation, hematuria, diarrhea, and vomiting, which raised an unconfirmed suspicion of acute arsenical poisoning.^9,10

Descartes is ranked alongside Benedict de Spinoza (1632–1677) and Gottfried Wilhelm Leibniz (1646–1716) as one of the great early rationalists. His intellectual independence and pursuit of truth had a lasting influence on Western thought, shaping the development of modern philosophy, science, and medicine.

His publications included:

• A Discourse on the Method of Correctly Conducting One’s Reason and Seeking Truth in the Sciences (heliocentric theory of the solar system including essays on Les Meteores, and La Dioptriques), 1637.
• Meditations on the first philosophy, 1641.
• La Geometrie, 1637.
• Principles of Philosophy, 1644.
• Passion of the Soul, 1649.
• Treatise on Man (De Homine) (De l’homme) written in the 1630s, published posthumously in 1662.
• Le Monde (the connection between body and soul), published posthumously in 1664.

End note

* By which he meant what we now call the nerve impulse, not the psyche.

References

1. Smith, Kurt. “Descartes’ Life and Works.” The Stanford Encyclopedia of Philosophy (Spring 2023 Edition). https://plato.stanford.edu/archives/spr2023/entries/descartes-works/
2. The Philosophical Works of Descartes. trans Elizabeth S. Haldane. Cambridge UP, 1911.
3. Descartes R. Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences (six parts), 1637. Edinburgh Sutherland and Knox 1853. Chapter 5. https://www.literature.org/authors/descartes-rene/reason-discourse/chapter-05.html
4. Westphal J. The Mind–Body Problem. MIT Press, 2016.
5. Vaccari A. Dissolving nature: How Descartes made us posthuman. Techné: Research in Philosophy and Technology, 2012;16(2):138-86.
6. Descartes R. De homine figuris et latinitate donatus a Florentio Schuyl. Leiden: P. Leffen and F. Moyard, 1662.
7. Watson RA. “René Descartes.” Encyclopedia Britannica, 27 Mar. 2025. https://www.britannica.com/biography/Rene-Descartes.
8. Philosopher Ad Absurdum. Descartes’ death: Descartes was murdered…or was he? September 12, 2021. https://philosopheradabsurdum.com/2021/09/12/descartes-was-murdered-or-was-he-the-death-of-rene-descartes-odd-histories/
9. Ebert T. La maladie mortelle de Descartes – pneumonie ou empoisonnment? Phil Papers, 2015. https://philarchive.org/archive/EBELMM
10. Fischer H. René Descartes found that Sweden was hazardous to his health. Hektoen International Winter 2024.