The Hume Society has chosen the topic of "naturalism" for the 2009 conference, and since a) I think I've got a line on naturalizing Hume's ethical theory, and especially since b) the conference is to be held in Halifax, Nova Scotia in August, my project this week is putting a submission together (not that that's an easy thing: if you check out their journal Hume Studies you'll see that these are indeed the preeminent Hume jocks, and a wonk I am not, although I've been a member for some years. But it's in Halifax so I have to give it a whack). So it's Saturday and I'm done with the kitchen and now I'll try blocking out the basic line here.
As most of my readers will know, Hume's best-known claim about ethical theory is that one cannot derive an "ought" from an "is." That is, descriptive propositions about the world do not as such contain any prescriptive meaning (it's important to remember that Hume is talking about propositions). Imperative prescriptions are motivated rather by "moral sentiments," non-cognitive feelings with which we have been endowed by nature, Hume knows not how. I think that Hume would be satisfied with Darwin's subsequent account of the etiology of ethical response. In fact Hume not only neatly flags the missing explanation, but asserts that the question is one for natural science, not philosophy (today I'm not going to provide any citations or passages, this is just off the top of my head). Part of Hume's bigger-picture agenda here is to debunk rationalist approaches to ethics, which the "schoolmen" took to involve ethics in transcendental metaphysics (Plato, by way of 17th century rationalism): Hume denies that ethical thinking is a branch of logic.
On my view, the most important thing to get about Hume is that he was not a skeptic, as is widely expounded. Rather Hume saw Cartesian skepticism as a pseudoproblem, and empiricism as the way out. Later the English phenomenalists of the early 20th century obscured this and now, with the aid of Wittgenstein, we are reconstructing Hume. Specifically Hume's is/ought distinction is frequently confounded with G. E. Moore's "naturalistic fallacy," which in fact is a very different sort of argument than Hume's. So if I can articulate the Humean critique of Cartesian skepticism, and show how Hume's is/ought distinction fits into that critique, this would help to nail down a persuasive account of Hume as a kind of naturalist about value.
Central to this is Hume's denial that it made any sense to speculate about an "external world" that might or might not be "corresponding" to the world of our perceptions. Experience cannot be distinguished from the world, and the world cannot be distinguished from experience. Thus there can be no question of "phenomenal properties," since any discussion of phenomenal properties is necessarily a discussion of experience, and any discussion of experience is necessarily a discussion of the world. There is no sense in which "my experience of blue" is different from "blue." To use Hume's language, mental contents consist of impressions and ideas, "impressions" being directly caused by interaction with the environment, "ideas" being fainter versions of impressions conjured by the mind during thought (e.g. memory).
The problem in interpretation of Hume's ethical theory is that often even people who grasp the anti-phenomenological import of Hume's empiricism don't interpret his ethical theory in a way that is consistent with his epistemology. When Hume says that there is no way to derive propositions about causality from propositions about correlation, or propositions about personal identity from propositions about self, he is saying no more nor no less than when he says this about propositions about value and propositions about fact. The is/ought distinction is simply another variation on the general theme that there is no need (no possibility) of "theory" over and above what is given by experience. And values (or rather, goodness and badness) are as much a given of experience as are causal relations and self-awareness. Thus Hume is not a "subjectivist" nor a "relativist" about ethics. In fact one of his targets is systematizers, for example of the religious variety, who claim that ethical responses are produced by embracing their systems, a form of cognitivism. The goodness and badness of experience is no more subjective or relative than is the blueness and redness of experience. On Hume's view there simply is no distinguishing between the "mental representation" and the "fact," and there are no exceptions to this general truth. Thus there is nothing to be said about "mental representation" nor about "facts," if either of these are taken to be somehow distinct from experience. Subjectivism is just as vacuous as realism on Hume's view.
Finally, notice that this is a general account of value, ethical and aesthetic. Arguing for the moral rightness of something is not different from arguing for the beauty of something. While we may feel queasy about this, realizing that the quality of experience is not always intersubjectively consistent, this is how it goes; there is no logical proof or refutation of the quality of experience. That does not mean that someone cannot learn to appreciate the goodness or the badness of something.