Plato: Five Dialogues: Euthyphro, Apology, Crito, Meno, Phaedo

by Plato

So now I do not know what virtue is; perhaps you knew [d] before you contacted me, but now you are certainly like one who does not know. Nevertheless, I want to examine and seek together with you what it may be. MENO: How will you look for it, Socrates, when you do not know at all what it is? How will you aim to search for something you do not know at all? If you should meet with it, how will you know that this is the thing that you did not know? [e] SOCRATES: I know what you want to say, Meno. Do you realize what a debater’s argument you are bringing up, that a man cannot search either for what he knows or for what he does not know? He cannot search for what he knows—since he knows it, there is no need to search—nor for what he does not know, for he does not know what to look for.

the famous Meno's paradox