For the third and final book in A Treatise of Human Nature, Hume discusses morals. He opposes the argument that virtue is a “conformity to reason” (508). Since reason is just a tool for determining what is true or false and is not a standard of truth or virtue in and of itself, it cannot prove whether or not an action is good or evil (510). Reason can only prove if a judgment behind an action was mistaken or factual, and we do not judge the morality of an action over whether or not the action was based on a correct or mistaken judgment.

Hume also argues more broadly against rationalism, reiterating his argument throughout A Treatise of Human Nature that we gain knowledge not through abstract reasoning, but by comparing ideas and gathering evidence through our senses. This also applies to morals. Patricide and incest are considered immoral, but it is not just because of reason. After all, animals are not capable of the same abstract reasoning as humans, but they too are capable of moral actions (519-20). Hume concludes with the observation that moral rules always seem to be written with “ought” and “ought not”, instead of “is” and “is not.