And as a supplementary incentive, notice that by dissolving and relocating the moral 'ought' in this way, Hume has solved the second key question about morality: "Now that I know how I morally ought to behave, what reason do I have to behave as I morally ought to behave? Hume's answer: "Because 'moral ought' is just a special case of 'practical ought'.

Despite being a fellow-traveler in these areas, I had no idea Hume actually laid out all these pieces. I'll have to go read some more Hume. I tend to defend it as straightforward application of Sidgwick's definition of ethics coupled with the actual English meaning of 'should', but clearly a good argument preceding that by a century or two would be even better.