How can virtue be learned? This is Meno's Paradox: what a man knows he cannot seek, since he knows it; and what he does not know he cannot seek, since he does not even know what to look for.
Socrates and Meno work through a number of possible definitions of virtue, each suggested by Meno and dismantled by Socrates. By the end of the dialogue, they still do not know what virtue is.
If virtue is knowledge, it must be teachable. But it cannot be teachable if there are no teachers of virtue.
Then what is it to follow after virtue, as Socrates asks us to do?
Read beforehand: http://www.gutenberg.org/files/1643/1643-h/1643-h.htm