Since the third century CE, Graeco-Roman philosophers assumed that only immaterial entities can be causally efficacious. Focusing on Alexander of Aphrodisias’ De fato, the project reconstructs his account of the respective roles of external causes and internal dispositions in animals’ and human action. It could be shown that the textual evidence does not support the prevailing interpretation that in Alexander’s view human beings differ from animals in their ability to act independently from their internal dispositions. Rather, the evidence suggests that according to Alexander, human beings shape the internal dispositions that lead, in turn, to action. In having this special ability, human beings are free agents, in Alexander’s view, because they not only initiate a course of events, but also determine its goal. This conclusion gives rise to the hypothesis that the assumption that only immaterial entities are causally efficacious was introduced in an attempt to secure teleologically successful causal chains.