Plato, Meno 80d

ἀπορεῖν. καὶ νῦν περὶ ἀρετῆς ὃ ἔστιν ἐγὼ μὲν οὐκ οἶδα, σὺ μέντοι ἴσως πρότερον μὲν ᾔδησθα πρὶν ἐμοῦ ἅψασθαι, νῦν μέντοι ὅμοιος εἶ οὐκ εἰδότι. ὅμως δὲ ἐθέλω μετὰ σοῦ σκέψασθαι καὶ συζητῆσαι ὅτι ποτέ ἐστιν. καὶ τίνα τρόπον ζητήσεις, ὦ Σώκρατες, τοῦτο ὃ μὴ οἶσθα τὸ παράπαν ὅτι ἐστίν; ποῖον γὰρ ὧν οὐκ οἶσθα προθέμενος ζητήσεις; ἢ εἰ καὶ ὅτι μάλιστα ἐντύχοις αὐτῷ, πῶς εἴσῃ ὅτι τοῦτό ἐστιν ὃ σὺ οὐκ ᾔδησθα;

I cause doubt in others. So now, for my part, I have no idea what virtue is, whilst you, though perhaps you may have known before you came in touch with me, are now as good as ignorant of it also. But none the less I am willing to join you in examining it and inquiring into its nature. Why, on what lines will you look, Socrates, for a thing of whose nature you know nothing at all? Pray, what sort of thing, amongst those that you know not, will you treat us to as the object of your search? Or even supposing, at the best, that you hit upon it, how will you know it is the thing you did not know? I understand the point you would make, Meno.