René Descartes

In brief

René Descartes (1596 to 1650) asked a question so simple it sounds childish and so radical it reorganized philosophy: is there anything at all I cannot doubt? His senses had deceived him, he could not always tell waking from dreaming, and for all he knew a powerful deceiver might be feeding him false beliefs about everything, even mathematics. So he resolved to doubt everything that could be doubted, to see whether anything survived. One thing did. However much he doubted, there had to be a thinker doing the doubting. "I think, therefore I am" (in Latin, cogito ergo sum) was the one belief that undid itself the moment you tried to deny it, because denying it is itself an act of thinking. On that single point he tried to rebuild the whole edifice of knowledge. Along the way he drew a sharp line between the mind, pure immaterial thought, and the body, a piece of clockwork governed by physics. That division, Cartesian dualism, gave science a free hand over the material world, but created a problem, how two such different things could interact, that has outlived every solution he offered.

The life

Descartes was born on 31 March 1596 in a small town in the Touraine region of France, later renamed Descartes in his honor. From about the age of ten he was educated at the Jesuit college of La Flèche, one of the best schools in Europe, where he received a thorough grounding in the scholastic philosophy descended from Aristotle, in classical languages, and in mathematics. He later wrote that he left his studies convinced of how little he actually knew, and struck that only mathematics offered certainty. He took a law degree at Poitiers in 1616 but never practiced.

Instead he traveled and soldiered. In 1618 he joined the army of Maurice of Nassau in the Dutch Republic. The decisive night of his life came in November 1619, quartered in a warm room in Germany, when he had a series of vivid dreams that he took as a sign that he was destined to found a unified science built on reason. Over the following years he did foundational work in mathematics, inventing what we now call Cartesian coordinates and, with them, analytic geometry, the bridge between algebra and shape that let equations describe curves. This was not a footnote to his philosophy: his conviction that the physical world is ultimately mathematical, that matter is just extension in space obeying exact laws, grew directly out of this work.

Around 1628 Descartes moved to the Dutch Republic, then the most tolerant place in Europe for a controversial thinker, and lived there for two decades. He had prepared a treatise on physics, The World, that assumed the Earth moves around the Sun; when he learned in 1633 that the Inquisition had condemned Galileo for exactly that claim, he suppressed it. What he published instead was more careful. The Discourse on the Method (1637), written in plain French rather than scholarly Latin, introduced both his method and the cogito. The Meditations on First Philosophy (1641), in Latin, was his masterpiece: six short chapters that stage the whole drama of doubt and reconstruction in the first person. He circulated it before publication to leading thinkers, including Thomas Hobbes and the theologian Antoine Arnauld, and printed their objections with his replies. The Principles of Philosophy (1644) tried to lay out a complete physics on the new foundations.

His most searching critic was not a professor but a princess. From 1643 Descartes corresponded with Princess Elisabeth of Bohemia, who pressed him with a question he could never answer: if the mind is immaterial and the body is matter, how does one move the other? In 1649 Queen Christina of Sweden invited him to Stockholm to tutor her. The northern winter, and her demand for lessons at five in the morning, wrecked a man used to lying abed thinking until noon. He caught pneumonia and died there on 11 February 1650.

The full treatment

The problem it answers

Descartes wrote at the moment the medieval picture of the world was collapsing. The authority of Aristotle, which had organized European thought for centuries, was crumbling under the new astronomy of Copernicus and Galileo. If the settled science of a thousand years could turn out to be false, what could anyone trust? Descartes wanted certainty as firm as a mathematical proof. He thought human knowledge had been built on unexamined assumptions absorbed in childhood, one resting on another, so a single rotten belief low in the pile could infect everything above it. His solution was drastic: tear the whole structure down, and rebuild only on foundations that could not be shaken.

How methodic doubt works

The method is doubt used as a tool, not despair. Descartes did not think our ordinary beliefs are mostly false; he suspended them deliberately to find what, if anything, could survive the harshest scrutiny. He proceeded in stages. First, the senses sometimes deceive us, and it is unwise to trust completely what has fooled you once. Second, there is no certain mark distinguishing waking from dreaming, which casts doubt on the entire physical world. Third, and most radical, he imagined an "evil demon," a being of great power and cunning who devotes all its energy to deceiving him, so that even the apparent truths of arithmetic and geometry might be planted falsehoods. This is the ancestor of every modern skeptical scenario, and its direct descendant is the-brain-in-a-vat: the worry that your entire experience could be an elaborate fabrication, with no way from the inside to tell.

The cogito

Under this maximal doubt, one thing holds. Suppose the demon deceives me about everything. Still, to be deceived I must exist to be deceived. However false my thoughts, there must be a thinker having them. "I think, therefore I am." The point is not an inference from a premise, as if existence were deduced from thinking; the certainty comes from the act itself. Every time I entertain the thought "I exist," I make it true in that instant, because doubting my existence is a form of thinking, and thinking requires a thinker. The demon cannot fool me here, for the very attempt to fool me confirms there is a me to be fooled. This is the fixed foundation Descartes had been hunting.

Building back the world

The cogito on its own is thin: it tells you that you exist as "a thing that thinks," but nothing about an external world. Descartes's reconstruction from that point is the part later philosophers found weakest. He argued that he possessed an idea of a perfect, infinite being (God) that could not have originated from an imperfect, finite mind like his own, so its cause must be an actually existing perfect being, who would not be a deceiver. Therefore, when he perceives something "clearly and distinctly," as he does the truths of mathematics, he can trust it, and on that guarantee he restored mathematics, the external world, and science. Critics from Arnauld onward complained that the reasoning circles: he uses clear and distinct perception to prove God, then uses God to certify clear and distinct perception. This "Cartesian circle" is one of the oldest objections in the book.

Mind and body

The doubt also delivered Descartes's most consequential and most troublesome doctrine. He could doubt whether he had a body, but could not doubt that he was thinking. From this he argued that mind and body must be two entirely different kinds of thing. The mind is a thinking, unextended substance (no size, shape, or location). The body is an extended, unthinking substance (pure matter, obeying the laws of physics like any machine). This is substance dualism, and it did real work. By declaring the body and all of nature to be mere mechanism, Descartes handed the physical world over to mathematical physics without remainder, and reserved a separate, immaterial realm for thought, the soul, and (he hoped) its immortality. The move suited both the new science and the old theology. But it left the union of the two a mystery that has dogged the theory ever since.

The interaction problem

If mind and body are utterly distinct, how do they touch? When I decide to raise my arm, a thought (immaterial, unextended) somehow sets a body (material, extended) in motion. Princess Elisabeth put the objection with unmatched clarity: everything we understand about causing motion involves contact and extension, so how could a thing with no extension push anything at all? Descartes's answer, that the interaction happens in the pineal gland, only relocates the puzzle, since a gland is still matter and the question of how thought moves matter returns intact. He never had a satisfying reply. The interaction problem is the permanent scandal of dualism.

Lineage

Descartes stands at the hinge between two worlds. Behind him is the scholastic tradition he was trained in and largely rejected, the Aristotelian philosophy taught in the universities (see aristotle), with its picture of substances defined by their purposes and a soul woven through the body. Descartes kept the vocabulary of "substance" but emptied nature of purpose, leaving only matter in motion. He also drew on ancient skepticism, turning it from a counsel of humility into a method of construction.

In front of him he opened the modern rationalist tradition. Baruch Spinoza (1632 to 1677) accepted Descartes's rigor but rejected the two-substance split, arguing there is only one substance with mind and matter as two aspects of it, which dissolves the interaction problem. Gottfried Wilhelm Leibniz (1646 to 1716) built another rationalist system on the trust in reason Descartes had licensed. Against them stood the British empiricists, above all john-locke and later David Hume, who denied Descartes's claim that the mind arrives stocked with innate ideas and insisted instead that all knowledge comes through the senses. That long argument between reason and experience is the subject of rationalism-and-empiricism, and Descartes is its opening figure on the rationalist side. The synthesis came from immanuel-kant, who argued both camps were half right: the mind supplies the forms of experience, but knowledge still requires the senses to fill them.

The strongest case for it

Descartes earns his title as founder of modern philosophy on method alone. He refused to argue from authority, and insisted that each person could and should think the foundations through for themselves, in the first person. That stance, that the individual reasoner is the court of last appeal, shaped everything that followed and is close to what first-principles-thinking now names. The cogito remains one of the few genuinely unshakable points in philosophy: two thousand years of skepticism have not dislodged it, and every attempt to deny it enacts it. His mechanization of nature was fruitful too, clearing the ground for the physics that followed. Much of modern epistemology and philosophy of mind is a series of answers to Descartes.

The strongest case against it

The objections are as old as the Meditations and as fresh as contemporary neuroscience. The reconstruction fails first. The theologian Antoine Arnauld (1612 to 1694), in the objections Descartes himself printed, pressed the Cartesian circle: the proof of God relies on trusting clear and distinct ideas, but trusting those ideas relies on the proof of God. If the circle cannot be broken, Descartes never earns his way back from the cogito to the external world.

The dualism fails next, and this is the deeper wound. Princess Elisabeth of Bohemia (1618 to 1680) identified the fatal difficulty already described: an unextended mind cannot intelligibly cause motion in an extended body. In the twentieth century Gilbert Ryle (1900 to 1976), in The Concept of Mind (1949), gave the mockery its enduring name, "the ghost in the machine," and argued that Descartes had committed a "category mistake," treating the mind as a ghostly thing of the same logical type as the body when talk of the mental is really talk about capacities and behavior. Most philosophers of mind since have been some flavor of materialist, holding that mental states are states of the physical brain, precisely to avoid Elisabeth's problem.

Empiricists rejected the starting materials. Locke and Hume denied that the mind comes furnished with the innate ideas Descartes leaned on (the idea of God, the axioms of geometry), insisting it begins as a blank sheet written on by experience. If there are no innate ideas, the argument for God loses its raw material.

And the method itself has been questioned. C. S. Peirce (1839 to 1914), founder of pragmatism, argued that Cartesian doubt is a pretense: you cannot actually doubt everything at once, and real inquiry always begins from beliefs we in fact hold, not from a foundation manufactured by fiat.

Where it stands now

The verdict is split cleanly down the middle of the work. Descartes's substance dualism is a minority position in professional philosophy of mind today; the interaction problem has driven most philosophers toward materialism. Yet the problems he bequeathed are more alive than ever. The relation between conscious experience and physical process, now debated as the-hard-problem-of-consciousness, is Descartes's mind-body question in modern dress. His skeptical scenario runs unbroken from the evil demon to the-brain-in-a-vat to the simulation hypothesis. The cogito still stands. And the first-person method, the decision to begin from one's own experience and demand reasons rather than authority, won so thoroughly that we barely notice it. Descartes lost most of his conclusions and won the argument about how to argue.

Test yourself

Try to doubt your own existence right now, sincerely, all the way down. The doubting is itself a thought, and a thought needs a thinker, so the attempt defeats itself. That is the cogito, felt from the inside rather than read about. Now the harder question Descartes could not answer: you are also, plainly, a body. When you decide to keep reading and your eyes move, what exactly is the point of contact between the deciding and the moving? If you cannot say, you have found the crack Princess Elisabeth found first.