Plato, Meno 75b

μή, ἀλλὰ σύ, ὦ Σώκρατες, εἰπέ. βούλει σοι χαρίσωμαι; πάνυ γε. ἐθελήσεις οὖν καὶ σὺ ἐμοὶ εἰπεῖν περὶ τῆς ἀρετῆς; ἔγωγε. προθυμητέον τοίνυν· ἄξιον γάρ. πάνυ μὲν οὖν. φέρε δή, πειρώμεθά σοι εἰπεῖν τί ἐστιν σχῆμα. σκόπει οὖν εἰ τόδε ἀποδέχῃ αὐτὸ εἶναι· ἔστω γὰρ δὴ ἡμῖν τοῦτο σχῆμα, ὃ μόνον τῶν ὄντων τυγχάνει χρώματι ἀεὶ ἑπόμενον. ἱκανῶς σοι, ἢ ἄλλως πως ζητεῖς; ἐγὼ γὰρ κἂν

No, it is you who must answer, Socrates. You wish me to do you the favour? By all means. And then you will agree to take your turn and answer me on virtue? I will. Well then, I must make the effort, for it is worth our while. Certainly. Come now, let me try and tell you what figure is. Just consider if you accept this description of it: figure, let us say, is the only existing thing that is found always following color. Are you satisfied, or are you looking for something different? I am sure I should be content with a similar account of virtue from you.