The following is based (very, very closely, since Hume almost puts it in a dialogue himself, and I've just colloquialized it a bit) on an argument in the first appendix to the Enquiry Concerning the Principles of Morals. H is Hume, A is the moral rationalist (who holds an abstract theory of morals like one finds in Malebranche or Clarke).
H: "Reason concerns itself with matters of fact and relations. Ask yourself, then, where is the matter of fact we call 'criminal ingratitude'. Point it out, determine the time of its existence, describe its essence or nature, explain the sense or faculty to which it discovers itself. Perhaps it resides in the mind of ungrateful person? If that's so, he must feel it and be conscious of it. But there is nothing there except ill-will or indifference. You cannot say that of themselves these are always and in all circumstances criminal. No; they are only criminal when directed towards people who have before expressed and displayed good-will towards us. So we can conclude that the moral crime of ingratitude is not any particular individual fact. Instead, it arises from a web of circumstances that excites the sentiment of blame in a spectator because of the structure and fabric of his mind."
A: "Your representation of me is false. Crime in this sense doesn't consist in a particular fact whose existence we know by reason. Instead, it consists in certain moral relations discovered by reason, in much the same way as we discover the truths of geometry or algebra."
H: "But what are these relations? In the case we are considering, I see first good-will and good-offices in one person, then ill-will and ill-offices in the other. The only relation between these is contrariety. Does the crime consist in that relation? But suppose a person bore me ill-will or did me ill-offices and I, in return, were indifferent towards him, or did him good-offices. Here is the same relation of contrariety; and yet my conduct is often highly laudable under these circumstances. Twist and turn this matter as much as you please, you can never base morality on relations. It must be based on sentiments.
When we say that two and three are equal to half of ten, I understand this relation perfectly. I conceive that if ten is divided in half, and one of these halves is compared to two added to three, the one will have as many units as the other. But when you use this as an analogy for moral relations, I confess that I don't understand you at all. A moral action, a crime like ingratitude, is a complicated object. Does the morality consist in the relation of its parts to each other? How? After what manner? Specify the relation: Be less vague, and you will easily see the falsehood of your claims."
A: "No, the morality consists in the relation of actions to the rule of right; they are called good or bad to the extent they agree or disagree with it. "
H: "What, then, is this rule of right? How is it determined? By reason, you say, which examines the moral relations of actions; these moral relations are determined by the comparison of actions to a rule; and that rule is determined by considering the moral relations of objects. Isn't this a fine way to reason!"
A: "But the fact that you have to get into such metaphysical reasoning as you do gives us a strong presumption that your claim is false."
H: "There is certainly metaphysical reasoning here, but all on your side. You are proposing an abstruse hypothesis, one which can never be made intelligible or squared with any particular example or illustration. Our hypothesis, on the other hand, is clear. We hold that morality is determined by sentiment. We define virtue to be any mental action or quality giving to a spectator the pleasing sentiment of approbation. Vice, of course, is the contrary. We then look at a plain matter of fact, to wit, what actions have this influence. Considering all the circumstances in which these actions agree, we endeavour to extract some general observations about these sentiments. If you call this metaphysics, and find any thing abstruse here, the natural conclusion is that your mind is not suited to the moral sciences."
