"The Thinker" by Rodin

Descartes was a French Catholic philosopher who rejected the scholasticism in which he had been trained. His chief concern was epistemology: how we know things. Descartes held that reason was the reliable way to know things. External authorities such as Scripture, popes, and theologians could not provide a starting point for knowledge. For this, Descartes would accept only "clear and distinct ideas" which should be as certain and demonstrable as mathematics. To arrive at these ideas, everything must be doubted. The innate idea, an intuition so basic that it could not be denied, was the 'thinking self' and the starting point of Descartes' philosophy: "I think, therefore I am." He reasoned his way from here to a rational proof of God's existence as a being 'greater than which cannot be thought.' Although Descartes was a devout man, his method of radical skepticism challenged traditional religious authority. Descartes' philosophy later informed deism, which sought to reduce religion to a few elemental truths.