Plato, Meno 95b

τί οὖν; ἐθέλουσιν οὗτοι παρέχειν αὑτοὺς διδασκάλους τοῖς νέοις, καὶ ὁμολογεῖν διδάσκαλοί τε εἶναι καὶ διδακτὸν ἀρετήν; οὐ μὰ τὸν Δία, ὦ Σώκρατες, ἀλλὰ τοτὲ μὲν ἂν αὐτῶν ἀκούσαις ὡς διδακτόν, τοτὲ δὲ ὡς οὔ. φῶμεν οὖν τούτους διδασκάλους εἶναι τούτου τοῦ πράγματος, οἷς μηδὲ αὐτὸ τοῦτο ὁμολογεῖται; οὔ μοι δοκεῖ, ὦ Σώκρατες. τί δὲ δή; οἱ σοφισταί σοι οὗτοι, οἵπερ μόνοι ἐπαγγέλλονται, δοκοῦσι διδάσκαλοι εἶναι ἀρετῆς;

Well then, are they willing to put themselves forward as teachers of the young, and avow that they are teachers and that virtue is to be taught? No, no, Socrates, I assure you: sometimes you may hear them refer to it as teachable, but sometimes as not. Then are we to call those persons teachers of this thing, when they do not even agree on that great question? I should say not, Socrates. Well, and what of the sophists? Do you consider these, its only professors,