This is a major new study of Thomas Aquinas, the most influential philosopher of the Middle Ages. The book offers a clear and accessible guide to the central project of Aquinas' philosophy: the understanding of human nature. Robert Pasnau sets the philosophy in the context of ancient and modern thought, and argues for some groundbreaking proposals for understanding some of the most difficult areas of Aquinas' thought: the relationship of soul to body, the workings of sense and intellect, the will and the passions, and personal identity. Structured around a close reading of the treatise on human nature from the Summa theologiae and deeply informed by a wide knowledge of the history of philosophy and contemporary philosophy, this study will offer specialists a series of novel and provocative interpretations, while providing students with a reference commentary on one of Aquinas' core texts.