This book develops a theory of the natural law that takes its orientation from the account of the natural law developed by Thomas Aquinas, as interpreted and supplemented in the context of scholastic theology in the twelfth and thirteenth centuries. Though this history might seem irrelevant to twenty-first-century life, Jean Porter shows that the scholastic approach to the natural law still has much to contribute to the contemporary discussion of Christian ethics. Aquinas and his interlocutors provide a way of thinking about the natural law that is distinctively theological while at the same time remaining open to other intellectual perspectives, including those of science. In the course of her work, Porter examines the scholastics' assumptions and beliefs about nature, Aquinas's account of happiness, and the overarching claim that reason can generate moral norms. Ultimately, Porter argues that a Thomistic theory of the natural law is well suited to provide a starting point for developing a more nuanced account of the relationship between specific beliefs and practices. While Aquinas's approach to the natural law may not provide a system of ethical norms that is both universally compelling and detailed enough to be practical, it does offer something that is arguably more valuable - namely, a way of reflecting theologically on the phenomenon of human morality.