In this commentary on Aristotle Physics book eight, chapters one to five, the sixth-century philosopher Simplicius quotes and explains important fragments of the Presocratic philosophers, provides the fragments of his Christian opponent Philoponus' Against Aristotle On the Eternity of the World, and makes extensive use of the lost commentary of Aristotle's leading defender, Alexander of Aphrodisias. This volume contains an English translation of Simplicius' important commentary, as well as a detailed introduction, explanatory notes and a bibliography.