René Descartes (31 March 1596 – 11 February 1650) was a French philosopher and mathematician, now widely considered to be the father of modern philosophy. He is counted among the rationalists, together with Spinoza and Leibniz, and was little concerned with observation or experiment. He was, however, the first to enunciate the principle of inertia, as also an early form of the principle of conservation of momentum. In his Principles of Philosophy (1643) he was also the first to advocate a fully relationalist account of motion. In mathematics, he founded the discipline of algebraic geometry, paving the way for Newton’s and Leibniz’s discovery of the calculus.
Descartes’ realionalism was tempered by a reliance, in his laws of collision, on the concept of rectilinear motion: indeed it seems likely that his espousal of relationalism was in part an attempt to evade the implication of his ‘vortex’ model of the solar system - that the Earth is in motion about the sun – the heresy for which Galileo was condemned by the Inquisition in 1633. Insisting instead that all motion was relational, and that as a result no unique state of motion could be attributed to any particle of matter, he proposed to count as ‘true’ or ‘philosophical’ motion the relative motion between adjacent particles of matter. Descartes also espoused the view that empty space or void was a contradiction in terms (that extension could only be a property -- indeed the defining property -- of matter).
In his Meditations on First Philosophy (1641) Descartes defended the primary-secondary quality distinction as grounded on reason alone, and argued for the existence of two kinds of substance: res extensa, matter, whose essential quality was extension, and res cogitans, thinking substance or mind, whose essential quality was thought. He also held that knowledge must be grounded on true and indubitable principles. This ground he claimed to have found in the proposition ‘cogito ergo sum’, ‘I think therefore I am’, a proposition that cannot fail to be true if thought or uttered, no matter how far the senses may be led into deception.
Descartes was born in La Haye en Touraine, Indre-et-Loire, France. He was educated at the Jesuit College of Royal Henry-Le-Grand at La Flèche, where he was introduced to mathematics and physics, including Galileo's work. His later life was spent in the Netherlands, where he composed all his major works.