René Descartes

Descartes was born in 1596 in La Haye en Touraine. His mother Jeanne Brochard died in 1597, shortly after a childbirth; his father Joachim was a counsellor in the Parlement of Brittany at Rennes. Because of his fragile health he began his schooling only in 1607, at about eleven years of age, at the Jesuit college of La Flèche.

At La Flèche he was introduced to mathematics and physics alongside scholastic philosophy. In 1616 he obtained a licence in canon and civil law at Poitiers, in accordance with his father's wishes. He chose instead a life of travel and military service, and later invested his property in bonds, from which he drew a sufficient income.

In 1618 Descartes joined the army of Maurice of Nassau at Breda, where he met Isaac Beeckman, who stimulated his interest in mathematical physics. In 1619 he travelled through Germany; according to his biographer Baillet, in the winter at Neuburg, in a heated room, he had a series of dreams that he took as a calling to a new science. Around 1629 he settled in the Dutch Republic.

His first major publication was the Discours de la méthode (1637), together with La Dioptrique, Les Météores and La Géométrie. In La Géométrie he applied algebra to geometry so that curves could be described with equations (analytic geometry). Later came the Meditationes (1641), the Principia philosophiae (1644) and Les passions de l'âme (1649).

Philosophically he sought an indubitable foundation for knowledge. Through methodical doubt he arrived at the cogito: 'I think, therefore I am'. He distinguished the thinking substance (res cogitans) from the extended, bodily substance (res extensa) and reduced the natural universe to mechanical processes. Princess Elisabeth of Bohemia put critical questions to him about the interaction between mind and body.

Descartes lived in the age of the Reformation and Counter-Reformation and of the Thirty Years' War (1618–1648). Although a Catholic, he took service in the Protestant Dutch States Army; in 1619 he may have joined the troops of the Duke of Bavaria, who belonged to the Catholic League. At the universities scholasticism based on Aristotle still dominated, while Galileo, Kepler and others developed new methods of natural science.

The Dutch Republic offered him tranquillity, a flourishing publishing market and relative tolerance; he lived in, among other places, Franeker, Amsterdam, Utrecht and Egmond. Even so he clashed with theologians: in Utrecht rector Gijsbert Voet condemned his follower Regius for teaching Descartes's physics, and in 1643 Cartesian philosophy was condemned there. He found support among friends such as Constantijn Huygens.

In 1633 he learned of the condemnation of Galileo by the Inquisition. He then left his own work Le Monde, which assumed a moving Earth, unpublished; it appeared only in 1664, after his death. He dedicated his Meditationes (1641) to the theologians of the Sorbonne.

Descartes' methodical doubt and his pursuit of certain knowledge changed the way philosophers thought about knowledge; they underlie rationalism, in which reason is the main source of knowledge. His mechanical description of nature, which explained phenomena from the motion and collision of matter, competed with Aristotelian thought and found followers at universities.

In mathematics he combined algebra and geometry in La Géométrie; his notation, with unknowns as x, y and z and powers as superscripts, later became standard. In optics he treated the refraction of light in La Dioptrique. Much of his natural philosophy proved incorrect, for example his vortex theory of planetary motion, which was displaced by Newton's mechanics.

Descartes died on 11 February 1650 in Stockholm of pneumonia, a few months after his arrival. He had agreed to tutor Queen Christina three times a week at five in the morning; other causes of death have also been proposed. He was buried in a cemetery for orphans; in 1819 his remains, without the skull and a finger, were reburied at Saint-Germain-des-Prés in Paris.

In 1663 the Catholic Church placed his works on the Index. The Cartesian coordinates are named after him (Cartesius is his Latinised name), but the modern system was developed in part by Pierre de Fermat and later mathematicians. His dualism of mind and body raised the mind–body problem, on which philosophers still disagree strongly.