Plato, Meno 96a

πειθόμενος μύθοισι σαόφροσιν. ἀλλὰ διδάσκων οὔ ποτε ποιήσεις τὸν κακὸν ἄνδρʼ ἀγαθόν. ἐννοεῖς ὅτι αὐτὸς αὑτῳ πάλιν περὶ τῶν αὐτῶν τἀναντία λέγει; φαίνεται. ἔχεις οὖν εἰπεῖν ἄλλου ὁτουοῦν πράγματος, οὗ οἱ μὲν φάσκοντες διδάσκαλοι εἶναι οὐχ ὅπως ἄλλων διδάσκαλοι ὁμολογοῦνται, ἀλλʼ οὐδὲ αὐτοὶ ἐπίστασθαι, ἀλλὰ πονηροὶ

for he would have followed the precepts of wisdom: but not by teaching wilt thou ever make the had man good You notice how in the second passage he contradicts himself on the same point? Apparently. Well, can you name any other subject in which the professing teachers are not only refused recognition as teachers of others, but regarded as not even understanding it themselves,