Plato, Meno 98b

καὶ μὴν καὶ ἐγὼ ὡς οὐκ εἰδὼς λέγω, ἀλλὰ εἰκάζων· ὅτι δέ ἐστίν τι ἀλλοῖον ὀρθὴ δόξα καὶ ἐπιστήμη, οὐ πάνυ μοι δοκῶ τοῦτο εἰκάζειν, ἀλλʼ εἴπερ τι ἄλλο φαίην ἂν εἰδέναι—ὀλίγα δʼ ἂν φαίην—ἓν δʼ οὖν καὶ τοῦτο ἐκείνων θείην ἂν ὧν οἶδα. καὶ ὀρθῶς γε, ὦ Σώκρατες, λέγεις. τί δέ; τόδε οὐκ ὀρθῶς, ὅτι ἀληθὴς δόξα ἡγουμένη τὸ ἔργον ἑκάστης τῆς πράξεως οὐδὲν χεῖρον ἀπεργάζεται ἢ ἐπιστήμη; καὶ τοῦτο δοκεῖς μοι ἀληθῆ λέγειν.

And indeed I too speak as one who does not know but only conjectures: yet that there is a difference between right opinion and knowledge is not at all a conjecture with me but something I would particularly assert that I knew: there are not many things of which I would say that, but this one, at any rate, I will include among those that I know. Yes, and you are right, Socrates, in so saying. Well, then, am I not right also in saying that true opinion leading the way renders the effect of each action as good as knowledge does? There again, Socrates, I think you speak the truth.