Plato, Meno 89c

ἆρα μαθήσει; δοκεῖ μοι ἤδη ἀναγκαῖον εἶναι· καὶ δῆλον, ὦ Σώκρατες, κατὰ τὴν ὑπόθεσιν, εἴπερ ἐπιστήμη ἐστὶν ἀρετή, ὅτι διδακτόν ἐστιν. ἴσως νὴ Δία· ἀλλὰ μὴ τοῦτο οὐ καλῶς ὡμολογήσαμεν; καὶ μὴν ἐδόκει γε ἄρτι καλῶς λέγεσθαι. ἀλλὰ μὴ οὐκ ἐν τῷ ἄρτι μόνον δέῃ αὐτὸ δοκεῖν καλῶς λέγεσθαι, ἀλλὰ καὶ ἐν τῷ νῦν καὶ ἐν τῷ ἔπειτα, εἰ μέλλει τι αὐτοῦ ὑγιὲς εἶναι.

is it by education? We must now conclude, I think, that it is; and plainly, Socrates, on our hypothesis that virtue is knowledge, it must be taught. Yes, I daresay; but what if we were not right in agreeing to that? Well, it seemed to be a correct statement a moment ago. Yes, but not only a moment ago must it seem correct, but now also and hereafter, if it is to be at all sound.