The traditional name 'destiny' was used in Hellenistic and Roman philosophy to designate a situation where everything happens in an inescapable order. Stoics were the main defenders of such theory. They maintained that, since everything has a cause, the sequence of causes and events is laid down beforehand and cannot be modified. This does not imply that men are not responsible of what they do, because human agency introduces in the world an irreducible kind of causality, different from the impulses of surrounding situation. Alexander criticises this theory. On his opinion, even if everything has a cause, not everything is necessary; besides, it is not possible to maintain, at the same time, that everything happens by necessity and there is something that depends on us. To understand that, he says, one must understand the causes in an Aristotelian way. Alexander's work is one of the main sources for the ancient debate on determinism; it can be of interest not only for the scholar interested in ancient philosophy, but also for the reader interested in contemporary philosophical debate about liberty and determinism. The reason is exactly the connection that Alexander established between freedom and our conception of causality.