Plato, Meno 80b

τὴν ψυχὴν καὶ τὸ στόμα ναρκῶ, καὶ οὐκ ἔχω ὅτι ἀποκρίνωμαί σοι. καίτοι μυριάκις γε περὶ ἀρετῆς παμπόλλους λόγους εἴρηκα καὶ πρὸς πολλούς, καὶ πάνυ εὖ, ὥς γε ἐμαυτῷ ἐδόκουν· νῦν δὲ οὐδʼ ὅτι ἐστὶν τὸ παράπαν ἔχω εἰπεῖν. καί μοι δοκεῖς εὖ βουλεύεσθαι οὐκ ἐκπλέων ἐνθένδε οὐδʼ ἀποδημῶν· εἰ γὰρ ξένος ἐν ἄλλῃ πόλει τοιαῦτα ποιοῖς, τάχʼ ἂν ὡς γόης ἀπαχθείης. πανοῦργος εἶ, ὦ Μένων, καὶ ὀλίγου ἐξηπάτησάς με. τί μάλιστα, ὦ Σώκρατες;

I feel my soul and my tongue quite benumbed, and I am at a loss what answer to give you. And yet on countless occasions I have made abundant speeches on virtue to various people—and very good speeches they were, so I thought—but now I cannot say one word as to what it is. You are well advised, I consider, in not voyaging or taking a trip away from home; for if you went on like this as a stranger in any other city you would very likely be taken up for a wizard. You are a rogue, Meno, and had almost deceived me. How is that, Socrates?