Aristotle’s account of voluntariness (to hekousion) lacks a sufficiently precise positive definition of ‘voluntary’. This is a problem: in Aristotle’s ethics, voluntariness is an important and unifying joint between psychological (character) and practical matters (action). I contend that Aristotle implicitly defines voluntariness as positive causal relation to an agent’s desire, where one’s character is the state of one’s faculty of desire. Since desires always have particular ends (final causes), a voluntary action is one which originates in the agent’s desire for (...) that action’s end. Using this interpretation, I answer questions about ‘mixed’ actions, culpable ignorance, and non-voluntariness. Without this interpretation, these questions cannot be systematically answered. (shrink)