What to read
David Hume. “A Dialogue.” In Essays and treatises on several subjects. 1758 (London).
Where can you find it? Eighteenth Century Collections Online [requires University of Chicago internet connection].
In general, you can find just about everything Hobbes, Hume, and Locke wrote in the InteLex PastMasters series, a searchable database on the web [requires University of Chicago connection]. Look under “British philosophy 1600-1900.” This can help with looking for particular phrases in the Treatise. However, it does not have this and the library’s copies all seem to be either checked out or in the Rare Books or microform collections. Since the copy on reserve is complete, I don’t see why you would bother anyway.
What to look for
Hume’s theory attempts to explain how human beings draw the moral distinctions that they do. It takes for granted, in other words, that we all draw pretty much the same moral distinctions and it is that phenomenon that calls for explanation.
Unfortunately, the members of our species are rather prone to disagreement about morality and do not draw the same moral distinctions: what one person calls good, another calls evil, and so on.
Hume does not come to grips with that in the Treatise, but he does in “A Dialogue.” Plus, the writing is unusually charming.