Aquinas' mature works, though theological in intent, contain much material which is philosophical in the sense that it is not in any way dependent on beliefs which are specifically Christian. His philosophical psychology, or philosophy of mind, was not taken seriously by secular thinkers, with one or two exceptions, from the 16th to the 19th centuries because of the dominance of ideas deriving from Descartes. In the present century many philosophers have come to regard the Cartesian system as quite exploded, and it can now be seen that Aquinas' philosophy of mind has a great contemporary interest and is indeed one of the best options in this area from any period. This book makes accessible those parts of Aquinas' system which are of enduring value. The kernal of the work is a close reading of the sections of "Summa Theologiae" which are devoted to human intellect and will and to the relationship between soul and body. It presupposes no knowledge of Latin or of medieval history, and relates Aquinas' system to a tradition of philosophy of mind inaugurated in the Anglo-American community by Wittgenstein and Ryle.