Like Hutcheson, Hume claims that morality originates not in reason alone but at least partly in sentiment. In this chapter, I do not present a detailed, comprehensive account – much less a defense – of Hume's arguments against moral rationalism. Instead, I sketch in very broad brushstrokes the overall structure of Hume's arguments against moral rationalism and how they fit with the parts of his philosophy that, in the next three chapters, I will examine more closely. There are three reasons I am covering this part of Hume's view so quickly. First, the secondary literature already contains a number of excellent discussions on this topic. Second, many of Hume's anti-rationalist arguments cover the same ground as Hutcheson's arguments in The Moral Sense, which I discussed in Chapter 12. And third, I think the other parts of Book III of the Treatise contain Hume's most important contributions to the philosophical developments I am charting in this study.

Almost all of Hume's arguments against moral rationalism fall into one of three categories: arguments based on the idea that reason alone cannot motivate, which I will, following Korsgaard (1986), call “arguments from motivational skepticism about practical reason”; arguments based on the idea that reason alone cannot provide the content necessary for moral judgments, which I will, once again following Korsgaard, call “arguments from content skepticism about practical reason”; and arguments based on the idea that sentimentalism constitutes a better explanation than rationalism of the observable moral phenomena.