Our feelings or sentiments produce our actions with the same degree of causal necessity, the same habitual expectation that the future will resemble the past, as that by which the rotation of the earth causes the sun to rise. So a proper science of human nature will account for human actions, as well as for human beliefs, by reference to the natural formation of habitual associations with human feelings. Clearly, rationality had no place in this account of morality. Although reason may judge relations of ideas and matters of fact, its most vivid outcomes never compel us to act as even the weakest of feelings may do. No compilation of facts, however complete or reliable, ever entails a moral obligation or results in action.
All human actions flow naturally from human feelings, without any interference from human reason.
It does not follow that all actions are of equal value. On Hume's view, the judgments and recommendations of traditional morality arise not from reason, but from a moral sense . Thus, we praise an instance of virtuous action precisely because it arouses in us a pleasant feeling, and we avoid committing a vicious action because we anticipate that doing so would produce pain. Our feelings provide a natural guide for moral conduct. Hume worked out the details of this account in Book III of the Treatise .