This article largely from: http://www.sfu.ca/~boland/methodology85.PDF and Morton, A. Practice in Philosophy, A. Morton, Blackwell Publishers, 1996
Grounds for Morality Having examined the epistemological basis for Hume's inductivism, we are ready to consider its application to human conduct. In morality as in all else, Hume supposed, our beliefs and actions are the products of custom or habit. Since all of our most scientific beliefs have exactly the same foundation, this account preserves the natural dignity of moral judgments.
Hume devoted the second book of the Treatise to an account of the human passions and a discussion of their role in the operation of the human will. It is our feelings or sentiments, Hume claimed, that exert practical influence over human volition and action. Observation does reveal a constant conjunction between having a motive (not a reason) for acting and performing the action in question. Hence, with the same reliability that characterizes our belief in any causal relation, on Hume's view, we further believe that our feelings have the power to result in actions.
At one level, of course, this entails that we are determined to act as we do. 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. (Like Locke, Hume denied that determination of this sort is relevant to our moral freedom; only when my actions are observed to be the effects of some cause outside myself could I decline to accept my own responsibility for them.) 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. "Reason is, and ought to be, only the slave of the passions," Hume held. All human actions flow naturally from human feelings, without any interference from human reason. In conjuction with Kantian ethics and Utilitarianism, Hume says feelings about the ideas are more important than the ideas alone.