This major new study of Thomas Aquinas, the most influential philosopher of the Middle Ages, offers a clear and accessible guide to the central project of Aquinas's philosophy--the understanding of human nature. Robert Pasnau sets the philosophy in the context of ancient and modern thought, and argues for groundbreaking proposals for understanding some of the most difficult areas of Aquinas's thought--the relationship of soul to body, the workings of sense and intellect, the will and the passions, and personal identity.