Agents are morally responsible for their actions only if they understand what they are doing. This much seems clear, but it is unclear exactly what agents must understand in order to be morally responsible; in other words, the epistemic condition for moral responsibility is difficult to discover. In this paper, I will investigate Aquinas’s discussion of knowledge, voluntariness, and moral responsibility in order to discover his views on this condition. Although he never provides a formal expression of such a condition, (...) I will use his discussions of related issues to construct a three-part epistemic condition for moral responsibility. In the process I will raise and discuss several interpretative difficulties, arguing that while some can be resolved, others, despite recent claims to the contrary, resist resolution. Finally, I will draw out several consequences of his account, noteworthy for a variety of reasons. (shrink)