Protagoras

"Πάντων χρημάτων μέτρον ἄνθρωπον εἶναι, τῶν μὲν ὄντων, ὡς ἔστι, τῶν δὲ μὴ ὄντων, ὡς οὐκ ἔστιν."

"When they [the Athenians] meet for a consultation on civic art, where they should be guided throughout by justice and good sense, they naturally allow advice from everybody, since it is held that everyone should partake of this excellence, or else that states cannot be."

"You, Socrates, began by saying that virtue cant be taught, and now you are insisting on the opposite, trying to show that all things are knowledge, justice, soundness of mind, even courage, from which it would follow that virtue most certainly can be taught."