Rationalist philosopher and mathematician, born in La Haye, France. Trained at the Jesuit College at La Fl�che, he remained a Catholic throughout his life, but soon became dissatisfied with scholasticism. While serving in the Bavarian army in 1619, he conceived it to be his task to refound human knowledge on a basis secure from scepticism. He expounded the major features of his project in his most famous work, the Meditationes de prima philosophia (1641, Meditations on First Philosophy). He began his enquiry by claiming that one can doubt all one's sense experiences, even the deliverances of reason, but that one cannot doubt one's own existence as a thinking being: cogito, ergo sum ("I think, therefore I am'). From this basis he argued that God must exist and cannot be a deceiver; therefore, his beliefs based on ordinary sense experience are correct. He also argued that mind and body are distinct substances, believing that this dualism made possible human freedom and immortality. His Discours de la mŽthode pour bien conduire sa raison, et chercher la vŽritŽ dans les sciences (1637, Discourse on the Method for Rightly Conducting One's Reason and Searching for Truth in the Sciences) contained appendices in which he virtually founded co-ordinate or analytic geometry, and made major contributions to optics. In 1649 he moved to Stockholm to teach Queen Christina of Sweden.