Themistius' treatment of Books 1-3 of Aristotle's Physics presents central features of Aristotle's thought about principles, causation, change and infinity. The tradition of synthesising and epitomising exegesis is here raised to a new level by the innovative method of paraphrase pioneered by Themistius. Taking selective, but telling, account of the earlier Peripatetic and Presocratic tradition, Themistius creates a framework that can still be profitably used in the study of Aristotle. This book will be of interest to all students of ancient and medieval philosophy, history and philosophy of science.