Plato, Meno 97c

ὅτι φρόνησις μόνον ἡγεῖται τοῦ ὀρθῶς πράττειν· τὸ δὲ ἄρα καὶ δόξα ἦν ἀληθής. ἔοικέ γε. οὐδὲν ἄρα ἧττον ὠφέλιμόν ἐστιν ὀρθὴ δόξα ἐπιστήμης. τοσούτῳ γε, ὦ Σώκρατες, ὅτι ὁ μὲν τὴν ἐπιστήμην ἔχων ἀεὶ ἂν ἐπιτυγχάνοι, ὁ δὲ τὴν ὀρθὴν δόξαν τοτὲ μὲν ἂν τυγχάνοι, τοτὲ δʼ οὔ. πῶς λέγεις; ὁ ἀεὶ ἔχων ὀρθὴν δόξαν οὐκ ἀεὶ ἂν τυγχάνοι, ἕωσπερ ὀρθὰ δοξάζοι; ἀνάγκη μοι φαίνεται· ὥστε θαυμάζω, ὦ Σώκρατες,

when we stated that knowledge is the only guide of right action; whereas we find there is also true opinion. So it seems. Then right opinion is just as useful as knowledge. With this difference, Socrates, that he who has knowledge will always hit on the right way, whereas he who has right opinion will sometimes do so, but sometimes not. How do you mean? Will not he who always has right opinion be always right, so long as he opines rightly? It appears to me that he must; and therefore I wonder, Socrates,