 Thomas Aquinas (1224/25–1274) is, by any measure, one of the most important, if not the most important, scholastic theologian of the later Middle Ages. Although his views on conscience and synderesis have received less attention in recent years than his theory of natural law, this is unjustified. For the scholastics, a full account of morality includes not only an explanation of how practical norms are grounded, but also an understanding of how they are grasped and how it is that rational agents, despite possessing an infallible guide to such knowledge, often fail to apply it. One’s metaethics, in other words, is inextricably linked to one’s moral psychology, which is to say, to one’s understanding of how the will and the intellect interact with one’s knowledge of right and wrong.