Towards the end of his life, Descartes published the first four parts of a projected six-part work, The Principles of Philosophy. This was intended to be the definitive statement of his complete system of philosophy, dealing with everything from cosmology to the nature of human happiness. Stephen Gaukroger examines the whole system, and reconstructs the last two parts, 'On Living Things' and 'On Man', from Descartes' other writings. He relates the work to the tradition of late Scholastic textbooks which it follows, and also to Descartes' other philosophical writings, and he examines the ways in which Descartes transformed not only the practice of natural philosophy but also our understanding of what it is to be a philosopher. His book is the first comprehensive examination of Descartes' complete philosophical system.