Until the launch of this series nearly twenty years ago, the 15,000 volumes of the ancient Greek commentators on Aristotle, written mainly between 200 and 600 AD, constituted the largest corpus of extant Greek philosophical writings not translated into English or other European languages. Over 40 volumes have now appeared in the series, which is planned in some 80 volumes altogether. 'The universe is, as it were, one machine, wherein the celestial spheres are analogous to the interlocking wheels and the particular beings are like the things moved by the wheels' and all events are determined by an inescapable necessity. To speak of free choice or self determination is only an illusion we human beings cherish. Thus writes Theodore the engineer to his old friend Proclus. Proclus' reply is one of the most remarkable discussions on fate, providence and free choice in Late Antiquity. It continues a long debate that had started with the first polemics of the Platonists against the Stoic doctrine of determinism. How can there be place for free choice and moral responsibility in a world governed by an unalterable fate? Notwithstanding its great interest, Proclus' treatise has not received the attention it deserves, probably because its text is not very accessible to the modern reader. It has survived only in a Latin medieval translation. This first English translation will bring the arguments he formulates again to the fore.