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 a 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 volume contains the first English translation of Themistius' commentary, accompanied by a detailed introduction, extensive explanatory notes and a bibliography.