Philoponus has been identified as the founder in dynamics of the theory of impetus, an inner force impressed from without, which, in its later recurrence, has been hailed as a scientific revolution. His commentary is translated here without the previously translated excursus, the Corollary on Void, also available in this series. Philoponus rejects Aristotle's attack on the very idea of void and of the possibility of motion in it, even though he thinks that void never occurs in fact. Philoponus' argument was later to be praised by Galileo. This volume contains the first English translation of Philoponus' commentary, as well as a detailed introduction, extensive explanatory notes and a bibliography.