This book is a detailed account of and commentary on Thomas Aquinas's most influential work: the Summa Theologiae. Intended for students and general readers interested in medieval philosophy and theology, the book will also appeal to professors and scholars, although it does not presuppose any previous knowledge of its subject. Following a scholarly account of Aquinas's life, the book explores his purposes in writing the Summa Theologiae and works systematically through each of its three Parts. It also relates their contents and Aquinas's teachings to that of other works and other thinkers both theological and philosophical. In addition to being expository, the volume aims to help readers think about the value of the Summa Theologiae for themselves. The concluding chapter considers the impact Aquinas's best-known work has had since its first appearance, and why it is still studied today. Davies's study is a solid and reflective introduction both to the Summa Theologiae and to Aquinas in general.