Descartes is rightly regarded as one of the inaugurators of the modern age, and there is no doubt that his thought profoundly altered the course of Western philosophy. In no area has this influence been more pervasive than in metaphysics and the philosophy of mind. But Descartes himself would perhaps have been surprised to learn that these aspects of his work were to be singled out by subsequent generations for special attention. For his own conception of philosophy, and of the philosophical enterprise he was engaged on, was enormously wide ranging; so far from being confined to ‘philosophy’ in the modern academic sense of that term, it had to do principally with what we should now call ‘science’. Descartes attempted, in his writings on cosmology, astronomy and physics, to develop a general theory of the origins and structure of the universe and the nature of matter, and he also did a considerable amount of detailed work in more specialized areas such as optics, meteorology, physiology, anatomy and medicine. In all these fields, Descartes aimed for explanatory economy; his goal was to derive all his results from a small number of principles of great simplicity and clarity, and he took mathematics as a model for the precise and unified structure of knowledge which he was seeking.