Anselm of Canterbury holds that freedom of the will is a necessary condition for moral responsibility. This condition, however, turns out to be trivially fulfilled by all rational creatures at all times. In order to clarify the necessary conditions for moral responsibility, we must look more widely at his discussion of the nature of the will and of willed action. In this paper, I examine his theory of voluntariness by clarifying his account of the sin of Satan in De casu (...) diaboli. Anselm agrees with Augustine that the sinful act cannot be given a causal explanation in terms of a distinct preceding act of will or desire or choice. He thus rejects volitionalist accounts of Satan's sin and thus of voluntary action in general. He moves beyond his predecessor, however, in insisting on the necessity of an explanation in terms of reasons, and his theory of the dual nature of the rational will is designed to meet this demand. A comparison of Satan's case with the case of the miser of De casu diaboli 3,... (shrink)