Plato’s Timaeus contains an argument that vice is involuntary. Here I present an interpretation of that argument and, upon doing so, relate the underlying conception of voluntariness to that found in Aristotle’s Eudemian Ethics. I argue that in the Timaeus, for something to be voluntary it must be caused by the agent’s intellect in a certain way. This idea, in turn, relies on an identification of the agent with her intellect: the reason that what is voluntary must be caused by (...) the agent’s intellect is that what is voluntary is what the agent herself is responsible for. The conception of the voluntary in the Eudemian Ethics differs less radically than one might think. There too Aristotle wishes to respect the idea that there is an important connection between what is voluntary and what is caused by our rational capacities, but he differs in how he fills out the relevant rational capacities. (shrink)