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.
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. As a straightforward matter of fact (discoverable by experience), virtue is always accompanied by a feeling of pleasure, and vice by a feeling of pain. 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. The ideas of benevolence, utility, and justice arouse our deepest and most pervasive feelings, he maintained, and these feelings in turn motivate us toward actions of moral worth. I offer assistance to those in need because it makes me feel good to do so, and I am fair in my dealings with others because it would make me feel bad if I were not. All of morality rests firmly upon the natural human inclination to seek pleasure and avoid pain.
This noncognitive derivation of morality from emotion rather than from reason may seem hopelessly subjective at first glance, but remember that on Hume's view our confidence in causal efficacy has a similar source. I do what is morally right in the same way that I believe there is an external world—by following my natural inclinations in the absence of rational evidence. Thus, Hume regarded himself as having provided morality with a status no less significant in human life than that of natural science.
Here are four paraphrased quotes from Hume:
1. Thought can neither justify or condemn an emotion, unless it is based on a false assumption or makes a mistake about which means will lead to which ends. It is not against reason to prefer the destruction of the whole world to my finger getting scratched.
2. Systems of morality usually begin with reasoning about matters of fact, for example about God or about human psychology. Then suddenly they switch, and instead of saying 'is' and 'is not' they begin to say 'ought' and 'ought not'. But premises about 'is' do not lead to conclusions about 'ought'.
3. My claim is that when two things repeatedly occur together, for example flame leading to heat, or an object's being solid and its having weight, then we become accustomed to expect one when the other appears. No one who has seen only one object move after another collides with it could deduce that all objects will when other objects collide with them. All the conclusions we draw from experience, therefore, are really the results of habit, not reasoning.
4. Imagine what it would be like if we went through a library keeping my philosophy in mind. Take any book, for example a theology book or an old-fashioned philosophy book. Ask:'Does it contain any reasoning about the physical world, based on evidence?' No. Burn it, then, for it can contain nothing but fraud and illusion.