Alexander of Aphrodisias, who flourished c. 200AD, was the leading Peripatetic philosopher of his age. Most of his philosophical energies were spent in commenting upon Aristotle: his commentary on the Prior Analytics remains one of the most thorough and helpful guides to this difficult work; in addition, the commentary preserves invaluable information about various aspects of Stoic logic, and it also presents a picture of categorical syllogistic at a turning point in its historical development. This volume contains a translation of the first third of the commentary - the part dealing with non-modal syllogistic. The translation is preceded by a substantial introduction which discusses Alexander's place in the commentatorial tradition and his use of logical terminology. The book is completed by a translation of the pertinent part of the Prior Analytics, a summary account of categorical syllogistic, and a set of indexes.