The aim of this chapter is to outline the foundations of Westermarck’s study of morality in three ways. First, I discuss Westermarck’s conception of ethics as a psychological and sociological discipline. Second, I explore the nature of Westermarck’s evolutionism by looking at the Darwinian psychological and biological roots of his theory-building. Correspondingly, Westermarck’s eclipse is linked to the decline of evolutionary theories of mind and behaviour in the twentieth-century social sciences. I conclude by showing how Westermarck’s methodological thinking was above all inspired by Darwin’s example of combining inductive and deductive reasoning.