Philoponus' commentary on the last part of Aristotle's Physics Book 4 does not offer major alternatives to Aristotle's science, as did his commentary on the earlier parts, concerning place, vacuum and motion in a vacuum. Aristotle's subject here is time, and his treatment of it had led to controversy in earlier writers. Philoponus does offer novelties when he treats motion round a bend as in one sense faster than motion on the straight over the same distance in the same time, because of the need to consider the greater effort involved. And he points out that in an earlier commentary on Book 8 he had argued against Aristotle for the possibility of a last instant of time. This volume contains an English translation of Philoponus' commentary, as well as a detailed introduction, extensive explanatory notes and a bibliography.