René Descartes

Key ideas

1. The method. In the Discourse on the Method Descartes sets four rules: accept nothing as true unless it is so clearly and distinctly known that it excludes all doubt; divide each difficulty into parts; proceed in order from the simplest to the more complex; and make enumerations so complete that nothing is omitted.
2. The method of doubt. In the Meditations he rejects whatever can be doubted, in order to find something certain. He gives three grounds: the senses sometimes deceive, waking experience may be indistinguishable from dreaming, and a powerful deceiver might make us wrong even about simple things.
3. The cogito. Even a deceiver cannot make him not exist while he thinks, so “I think, therefore I am” (cogito, ergo sum) is the first certain truth. What he is, he concludes, is a thing that thinks.
4. Clear and distinct perception and God. Descartes takes whatever he perceives very clearly and distinctly to be true, and argues that God exists and is no deceiver, first from the idea of an infinite being in Meditation III and then, in Meditation V, from the idea of a perfect being. Arnauld objected that the argument is circular.
5. Mind–body dualism. Mind, whose essence is thought, and body, whose essence is extension, are really distinct substances, since he can doubt that he has a body but not that he thinks. How they interact, which Princess Elisabeth of Bohemia pressed, he admitted he had no good answer to; in the Passions of the Soul he placed the mind’s chief seat in the pineal gland.
6. Mechanism and the animal-machine. Nature is matter in motion under quantitative laws, and matter is extension. Animals are machines without minds, and in the Discourse he proposes two tests, language use and general reason, by which a human being could always be told from a machine that imitated one.
7. Analytic geometry. In La Géométrie (1637) he joined algebra and geometry, so that curves can be described by equations. Cartesian coordinates are named after him.

René Descartes (1596–1650) was a French philosopher, mathematician and scientist who made doubt the starting point of modern philosophy. He is the author of “I think, therefore I am”, of the view that mind and body are distinct kinds of thing, and of analytic geometry, which is why Cartesian coordinates carry his name. With Spinoza and Leibniz he is counted among the three great rationalists of early modern philosophy.

What do we know of his life?

Descartes was born on 31 March 1596 at La Haye in Touraine. He studied at the Jesuit college of La Flèche, where he learned the scholastic Aristotelian philosophy he later rejected, and took a law degree at Poitiers in 1616. In 1618 he went to Breda as a gentleman soldier and met Isaac Beeckman; on the night of 10 November 1619, in the traditional account, three dreams gave him the task of reforming all knowledge. From the end of 1628 he lived in the Dutch Republic. When he learned that the Inquisition had condemned Galileo for maintaining heliocentrism, he withheld The World, his treatise on physics.

The Discourse on the Method appeared in 1637 in French, with essays on optics, meteorology and geometry. The Meditations (1641) and the Principles of Philosophy (1644) were in Latin, the Passions of the Soul in French in 1649. In the mid-1640s he corresponded with Princess Elisabeth of Bohemia. He accepted an invitation from Queen Christina of Sweden in 1649, and died of pneumonia at Stockholm on 11 February 1650.

How does the method of doubt work?

Descartes wants a foundation, a first certainty on which knowledge can be rebuilt, so in the Meditations he rejects whatever can be doubted. The senses deceive; waking life may be a dream; and a powerful deceiver, the evil demon, might be misleading him even about arithmetic. This is scepticism used as a tool, in the service of what is now called foundationalism.

What does the cogito show?

Doubt cannot reach the doubter. In Molyneux’s 1680 translation, “whenever this sentence I am, I exist, is spoken or thought of by Me, ‘tis necessarily True”. In the Discourse he writes, in Veitch’s translation, “I think, therefore I am”, and takes it as the first principle of his philosophy. The cogito establishes that he exists as a thinking thing, and that his own mind is better known than any body. The library has Part IV of the Discourse and the piece of wax from Meditation II, with a commentary on why its remark that seeing is often judging matters for my research on AI.

Why does God matter to his system?

The cogito is a single certainty. To trust more, Descartes takes whatever is perceived clearly and distinctly to be true, and tries to show that this rule is guaranteed. In Meditation III he argues that the idea of an infinite being could only have an infinite cause; in Meditation V he argues that existence belongs to the essence of a perfect being (the ontological argument). A perfect God would not deceive. Arnauld objected that this reasons in a circle, and Descartes replied that God guarantees our memory of demonstrations, not the clear perception before us now.

What is the relation of mind to body?

Descartes argues for a real distinction between mind, whose essence is thought, and body, whose essence is extension: he can doubt that he has a body but not that he thinks. This is dualism, and it made the relation of mind to body the central problem of the field. Princess Elisabeth asked how an unextended mind could move an extended body; Descartes admitted he lacked a good answer, and in the Passions of the Soul located the mind’s principal seat in the pineal gland. Gilbert Ryle coined “the ghost in the machine” to criticise the view.

His physics is mechanical. Matter is extension; there are no substantial forms; animals are machines without minds. In Discourse V he asks how we could tell a machine that imitated a human body from a human being, and gives two tests: it could never arrange words to reply to whatever is said in its presence, as the dullest people can, and it would fail at some tasks because it acts from the arrangement of its organs, whereas reason is “an universal instrument”. The tests resemble, in outline, the Turing test.

What came after him?

Spinoza and Leibniz both responded to Cartesian dualism, and Malebranche’s occasionalism tried to solve the interaction problem. Locke took over his “way of ideas” but rejected innate ideas, which he thought Descartes’s view entailed. In mathematics, analytic geometry made the discovery of calculus easier.

How should you start reading Descartes?

Begin with Part IV of the Discourse, which gives the cogito in a few pages, and Part V for the machine tests. Then read Meditations I–II, in which doubt and the cogito are worked out in detail. Veitch’s translations of the Discourse and the Meditations are free; the 1680 English Six Metaphysical Meditations, which includes Hobbes’s objections and Descartes’s answers, is on Project Gutenberg.