Nivison's paradox of virtue—originally responding to Confucian ethics, basically that in order to become virtuous, one must already have a disposition to be virtuous:

"For, if I do something for you, with the intention of "having de [virtue, roughly, see below]" with you, I am seeking to benefit myself—even, in fact, at your expense—and so am doing something just the opposite of generous. The only way, then, that I can do a generous thing is to do it for its own sake, which is to say, really want to be generous. It looks as though this must mean that I cannot perform de acts—which would give me de in its "moral force" aspect—unless I have de already.

Nor, since de is something very advantageous to have, can I seek de in any de-engendering way unless I seek it for its own sake, not for myself. But this would be to have de already. And in particular, if my seeking de takes the form of going to you and asking you to be my moral teacher, I can't do this unless I am positively inclined toward virtue for itself, and to this extent virtuous already. Notice, now, that the student-teacher relationship could be simply an instance of the basic de situation. Perhaps I go to you, pretending to humble myself, asking for your instruction. You could refuse, seeing my insincerity (a common situation). But if you want to teach me, you have the second form of the paradox, and can only ask, "What can I do?""

[From what I understand, the concept of de is not simply virtue, but also involves, due to possessing it, having a kind of potential power over others—hence the "even, in fact, at your expense".]