René Descartes

René Descartes was a French philosopher, mathematician and scientist, often called the father of modern philosophy. Seeking a foundation for knowledge that no doubt could shake, he found it in the certainty of his own thinking, summed up in the phrase "I think, therefore I am". He also invented the coordinate geometry that joins algebra and geometry, and proposed a mechanical picture of the physical world that dominated European science until Isaac Newton.

Descartes was born on 31 March 1596 at La Haye en Touraine, in the Kingdom of France; the town is now named Descartes after him. His family belonged to the minor nobility of office, and his father was a councillor in the parlement of Brittany. His mother died when he was about a year old, and he was raised largely by his grandmother.

From around 1607 he studied at the Jesuit college of La Flèche, one of the best schools in France, where he learned classical languages, mathematics and the philosophy of Aristotle as taught by the scholastics. He admired the certainty of mathematics but felt that the rest of his education had left him with more doubts than knowledge. In 1616 he took a law degree at the University of Poitiers, but never practised.

In 1618 Descartes went to Breda in the Dutch Republic to serve as an unpaid gentleman volunteer in the army of Maurice of Nassau. There he met the Dutch scientist Isaac Beeckman, who encouraged him to apply mathematics to physical problems. He then travelled to Germany at the start of the Thirty Years' War and joined the army of Maximilian of Bavaria. On the night of 10 November 1619, shut in a heated room in a town on the Danube, he had a series of vivid dreams which he took as a call to found a new, unified science based on mathematical method. He spent much of the 1620s travelling and in Paris, where he worked on optics and mathematics and came to know the friar Marin Mersenne, who became his main correspondent.

In 1628 or 1629 Descartes moved to the Dutch Republic, where he lived for most of the next twenty years, changing address often to protect his privacy. He completed a treatise on physics, The World, but withdrew it from publication in 1633 when he learned that Galileo Galilei had been condemned by the Roman Inquisition, since it also assumed that the earth moves. In 1635 a servant, Helena Jans van der Strom, gave birth to his daughter Francine, who died of fever in 1640, a loss that deeply affected him.

His first published book, the Discourse on Method (1637), was written in French so that ordinary readers could follow it. It introduced three scientific essays, on optics, meteorology and geometry. The Meditations on First Philosophy followed in Latin in 1641, printed with objections from leading thinkers including Thomas Hobbes, Pierre Gassendi and Antoine Arnauld, and his replies. The Principles of Philosophy (1644) set out his whole system as a textbook. His views caused bitter disputes with Protestant theologians at Utrecht and Leiden.

In the Meditations, Descartes resolves to doubt everything that can possibly be doubted, so that whatever survives can serve as a secure foundation. The senses sometimes deceive; he might be dreaming; he might even be fooled by an all-powerful evil demon into believing false things about mathematics. Yet however far the doubt is pushed, he cannot doubt that he himself is doubting, and therefore thinking. His own existence as a thinking thing is certain whenever he considers it.

From this starting point he argues for the existence of a perfect God, who would not deceive him, and then uses God's reliability to restore confidence in clear and distinct ideas and, with care, in the existence of the physical world. Critics soon objected that this reasoning is circular, since he seems to rely on clear ideas to prove God and on God to justify clear ideas. This problem, known as the Cartesian circle, is still debated.

Descartes held that reality contains two fundamentally different kinds of substance: mind, whose essence is thought, and matter, whose essence is extension, or taking up space. This view, called dualism, implies that the soul can exist without the body. It also raised a famous problem: how can an unextended mind move a physical body and be affected by it? He suggested that the two interact in the pineal gland of the brain, an answer few found satisfying.

In nature, he argued, everything except the human mind can be explained mechanically, by the size, shape and motion of particles. Animals were in his view complex machines. His physics explained the planets as carried around by vortices of fine matter. Much of it was later replaced by Newton's theory of gravitation, but his ideal of a mathematical, mechanical science, and his law of inertia, shaped the Scientific Revolution. In the Geometry he showed how curves could be described by algebraic equations using coordinates, the basis of what are still called Cartesian coordinates.

Descartes corresponded with Princess Elisabeth of Bohemia, who pressed him on the mind-body problem, and for her he wrote The Passions of the Soul (1649), on the emotions. In 1649 he accepted an invitation from Queen Christina of Sweden and went to Stockholm, where he was asked to tutor her early in the morning. He fell ill in the cold winter and died on 11 February 1650, almost certainly of pneumonia. A theory that he was poisoned has been proposed but is unproven. His remains were later moved to France, and most of them now lie in the church of Saint-Germain-des-Prés in Paris.

Descartes's ideas spread quickly, and Cartesianism became the leading new philosophy in France and the Netherlands, though the Catholic Church placed his works on its Index of forbidden books in 1663. Baruch Spinoza, Blaise Pascal and Gottfried Wilhelm Leibniz all developed their own systems in dialogue with his, while John Locke took a more empiricist path. His method of doubt, his focus on the knowing subject and his split between mind and matter defined the agenda of early modern philosophy and remain points of reference in debates on consciousness. He is one of the central figures in the intellectual history of France.