This book is the first study of the ontological system of Alexander of Aphrodisias (floruit c. 200 AD), famous for his commentaries on the works of Aristotle. By drawing not only on the entire known corpus of the commentator's works, but also on numerous new Greek and Arabic sources, Marwan Rashed aims at defining Alexander’s place in the history of metaphysics. Alexander’s attempt to substantiate the objectivity of the Aristotelian form draws down the curtain on the phase of the Hellenistic peripatos, at the same time marking the beginning of medieval Aristotelianism.