Descartes was a French philosopher, the “father of modern philosophy”, but also a mathematician and a scientist. His early work on physics, Le Monde (“The World”), included a heliocentric view of the solar system, and Descartes himself sought to supress it on learning of Galileo’s condemnation by the Roman Inquisition for expressing similar views.
Descartes‘ main legacy to philosophy is his radical scepticism concerning the world around us. The experience of our senses, Descartes argued, can be equally well explained by the hypothesis that we are dreaming as by the hypothesis that they accurately represent a world that exists around us. As we cannot disprove the former hypothesis, we cannot know that the latter is the case.
Descartes had an answer to this sceptical argument: Though we may sometimes fall into error in our reasoning, he suggested, we will not do so if we accept only those truths that we clearly and distinctly perceive. For our rational faculties are, he claimed, given to us by God, and God is no deceiver. He thus made knowledge of the existence of God the foundation of all empirical knowledge.
Descartes‘ rebuttal of scepticism has been far less well received than his argument for it. Quite apart from anything else, it rests on belief in God. Further, this belief in God cannot be justified by any a posteriori argument, any argument using empirical observation, for until God’s existence is proven empirical observation cannot be trusted. Descartes was unperturbed by these problems, for he thought that the existence of God could be known a priori, independent of any empirical observation, via a form of ontological argument.