How do we determine whether an action is right or wrong? Until recently, philosophers assumed that this question could be answered by means of a theory of morality, which set forth apodictic rules for moral behaviour. More recently, however, a number of philosophers have questioned whether there can be a theory of morality in quite this sense. Jean Porter is sympathetic to their critiques of moral theories, but questions whether these go far enough in offering a positive alternative to a modern view of the moral act. She argues that the work of Aquinas offers an alternative account of moral rationality, in terms of which moral reasoning is understood as dialectical rather than deductive, and questions are resolved in a wider contect of ethical thought. The Thomist account of the moral virtues and prudence is seen to offer unexpected insights into the relationship between moral rules and the practice of the virtues, and so contributes to our own moral reflection.