This book examines Aristotle's metaphysics and his account of nature, stressing the ways in which his desire to explain observed natural processes shaped his philosophical thought. It departs radically from a tradition of interpretation, in which Aristotle is understood to have approached problems with a set of abstract principles in hand, principles derived from critical reflection on the views of his predecessors. This is a major reevaluation of Aristotle's metaphysics that will interest philosophers, classicists, and historians of classical science.