I'll have to go read some more Hume.

Try this

And, indeed, to drop all figurative expression, what hopes can we ever have of engaging mankind to a practice, which we confess full of austerity and rigour? Or what theory of morals can ever serve any useful purpose, unless it can show, by a particular detail, that all the duties, which it recommends, are also the true interest of each individual? The peculiar advantage of the foregoing system seems to be, that it furnishes proper mediums for that purpose.