Meno

may not the art of which neither teachers nor disciples exist be assumed to be incapable of being taught

do not retract the assertion that if virtue is knowledge it may be taught; but I fear that I have some reason in doubting whether virtue is knowledge

And is not this universally true of human nature? All other things hang upon the soul, and the things of the soul herself hang upon wisdom, if they are to be good; and so wisdom is inferred to be that which profits—and virtue, as we say, is profitable