One of the arguments in Aristotle's On the Heavens propounds that the world neither came to be nor will perish. This volume contains the pagan Neoplatonist Simplicius of Cilicia's commentary on the first part of this this important work. The commentary is notable and unusual because Simplicius includes in his discussion lengthy representations of the Christian John Philoponus' criticisms of Aristotle along with his own, frequently sarcastic, responses.This is the first complete translation into a modern language of Simplicius' commentary, and is accompanied by a detailed introduction, extensive explanatory notes and a bibliography.