Descartes' attempt to ground the possibility of human knowledge in the existence of God was judged to be a complete failure by his contemporaries. This remains the universal opinion of philosophers to this day, despite the fact that three and a half centuries of secular epistemology which attempts to ground the possibility of knowledge either in the unaided human intellect or in natural processes has failed to do any better. Further, the leading twentieth century attempts at theistic epistemology reject both the conception of knowledge and the standards of epistemic evaluation that Descartes takes for granted. In this book - partly an interpretation of Descartes and partly an attempt to complete his project the author endeavours to show that a theistic epistemology incorporating Platonic and Aristotelian/Thomist elements can revitalize the Cartesian approach to the solution of the central problems of epistemology, including that most elusive of prizes the proof of the external world. This book is essential reading for students of epistemology, metaphysics and philosophy.