The paper examines Simplicius's doctrine of propensities (epitedeioteis) in his commentary on Aristotle's Categories and follows its application by the late thirteenth century theologian and philosopher James of Viterbo to problems relating to the causes of volition, intellection and natural change. Although he uses Aristotelian terminology and means his doctrine to conflict minimally with those of Aristotle, James's doctrine of propensities really constitutes an attempt to provide a technically rigorous dressing to his Augustinian and Boethian convictions. Central to James's procedure is his rejection, following Henry of Ghent, of the principle that “everything that is moved is moved by another”. James uses Simplicius' doctrine of propensities as a means of extending the rejection of that principle, which Henry had limited to the case of the will, to cognitive operations and natural change. The result is a theory of cognition and volition that sees the soul as the principal cause of its own acts, and a theory of natural change that minimizes the causal impact of external agents.