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(1596–1650)

French mathematician and founding father of modern philosophy. Born in La Haye, near Tours, Descartes was educated at the new Jesuit college at La Flèche, before reading law at Poitiers. In 1618 he enlisted at his own expense in the Dutch army of Maurice of Nassau, in order to have the leisure to think. His interest in the methodology of a unified science is supposed to have been stimulated by a dream ‘in a stove-heated room’ when he was serving at Ulm in 1619. In the subsequent ten years he travelled widely, returning to Holland in 1628. Little is known of his private life, but the death of his illegitimate five-year-old daughter Francine in 1640 is known to have been a devastating blow. His first work, the Regulae ad Directionem Ingenii (1628/9), was never completed. In Holland, between 1628 and 1649, Descartes first wrote, and then cautiously suppressed, Le Monde (1634), and in 1637 produced the Discours de la méthode as a preface to the treatise on mathematics and physics in which he introduced the notion of Cartesian co-ordinates. His best-known philosophical work, the Meditationes de Prima Philosophia (Meditations on First Philosophy ), together with objections by distinguished contemporaries and replies by Descartes (the Objections and Replies ), appeared in 1641. The authors of the objections are: first set, the Dutch theologian Johan de Kater; second set, Mersenne ; third set, Hobbes ; fourth set, Arnauld ; fifth set, Gassendi ; and sixth set, Mersenne . The second edition (1642) of the Meditations included a seventh set by the Jesuit Pierre Bourdin. Descartes's penultimate work, the Principia Philosophiae (Principles of Philosophy ) of 1644 was designed partly for use as a theological textbook. His last work was Les Passions de l’âme (The Passions of the Soul ), published in 1649. In that year Descartes visited the court of Kristina of Sweden, where he contracted pneumonia, allegedly through being required to break his normal habit of late rising in order to give lessons at 5.00 a.m. His last words are supposed to have been ‘Ça, mon âme, il faut partir’ (‘so, my soul, it is time to part’).

Descartes's theory of knowledge starts with the quest for certainty, for an indubitable starting-point or foundation on the basis alone of which progress is possible (see method of doubt ). This is eventually found in the celebrated ‘Cogito ergo sum’: I think therefore I am. By locating the point of certainty in my own awareness of my own self, Descartes gives a first-person twist to the theory of knowledge that dominated the following centuries in spite of various counter-attacks on behalf of social and public starting-points. The metaphysics associated with this priority is the famous Cartesian dualism, or separation of mind and matter into two different but interacting substances. Descartes rigorously and rightly sees that it takes divine dispensation to certify any relationship between the two realms thus divided, and to prove the reliability of the senses invokes a ‘clear and distinct perception’ of highly dubious proofs of the existence of a benevolent deity. This has not met general acceptance: as Hume drily puts it, ‘to have recourse to the veracity of the supreme Being, in order to prove the veracity of our senses, is surely making a very unexpected circuit’.

In his own time Descartes's conception of the entirely separate substance of the mind was recognized to give rise to insoluble problems of the nature of the causal connection between the two (see occasionalism ). It also gives rise to the problem, insoluble in its own terms, of other minds . Descartes's notorious denial that non-human animals are conscious is a stark illustration of the problem. In his conception of matter Descartes also gives preference to rational cogitation over anything derived from the senses. Since we can conceive of the matter of a ball of wax surviving changes to its sensible qualities, matter is not an empirical concept, but eventually an entirely geometrical one, with extension and motion as its only physical nature. Descartes's thought here is reflected in Leibniz's view, held later by Russell, that the qualities of sense experience have no resemblance to qualities of things, so that knowledge of the external world is essentially knowledge of structure rather than of filling. On this basis Descartes erects a remarkable physics. Since matter is in effect the same as extension there can be no empty space or void ; since there is no empty space motion is not a question of occupying previously empty space, but is to be thought of in terms of vortices (like the motion of a liquid).

Although the structure of Descartes's epistemology, theory of mind, and theory of matter have been rejected many times, their relentless exposure of the hardest issues, their exemplary clarity, and even their initial plausibility, all contrive to make him the central point of reference for modern philosophy.