French rationalist philosopher and mathematician, "the father of modern philosophy," born at La Haye, near tours; trained at the Jesuit College at La Fleche, he remained a Roman Catholic throughout his life but soon became dissatisfied with scholasticism. While serving in the Bavarian army in 1619, he became convinced that it was his calling to refound human knowledge on a basis secure from skepticism. He expounded this idea in his most famous work, Meditationes de Prima Philosophia (Meditations on First Philosophy, 1641). He began his inquiry by claiming that while one can doubt all the experiences of one's senses and the fruits of one's reason, one cannot doubt one's own existence as a thinking being, summarized in the Latin expression for which Descartes is famous: Cogito ergo sum (I think therefore I am). From this basis he argued that God must exist and cannot be a deceiver, therefore it is possible to discern truth through one's senses (a fundamental premise of rationalism). Descartes also argued that mind and body are distinct substances, believing that this dualism made possible human freedom and immortality. These ideas are seen in his Discours de la methode pour bien conduire sa raison, et chercher la verita dans les sciences (Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences, 1637). In 1649, he moved to Stockholm to begin teaching Queen Christina of Sweden, but died of pneumonia the following year.

Descartes' famous statement "Cogito ergo sum" — "I think, therefore I am" — and his argument for the existence of God are to be found in his Principles of Philosophy (1644), selections of which were translated into English by Professor John Veitch and are published here.

• Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences (1637)
• Meditations on First Philosophy (1641)
• Selections from Principles of Philosophy (1644)