Plato, Meno 97b

τί δʼ εἴ τις ὀρθῶς μὲν δοξάζων ἥτις ἐστὶν ἡ ὁδός, ἐληλυθὼς δὲ μὴ μηδʼ ἐπιστάμενος, οὐ καὶ οὗτος ἂν ὀρθῶς ἡγοῖτο; πάνυ γε. καὶ ἕως γʼ ἄν που ὀρθὴν δόξαν ἔχῃ περὶ ὧν ὁ ἕτερος ἐπιστήμην, οὐδὲν χείρων ἡγεμὼν ἔσται, οἰόμενος μὲν ἀληθῆ, φρονῶν δὲ μή, τοῦ τοῦτο φρονοῦντος. οὐδὲν γάρ. δόξα ἄρα ἀληθὴς πρὸς ὀρθότητα πράξεως οὐδὲν χείρων ἡγεμὼν φρονήσεως· καὶ τοῦτό ἐστιν ὃ νυνδὴ παρελείπομεν ἐν τῇ περὶ τῆς ἀρετῆς σκέψει ὁποῖόν τι εἴη, λέγοντες

Well, and a person who had a right opinion as to which was the way, but had never been there and did not really know, might give right guidance, might he not? Certainly. And so long, I presume, as he has right opinion about that which the other man really knows, he will be just as good a guide—if he thinks the truth instead of knowing it—as the man who has the knowledge. Just as good. Hence true opinion is as good a guide to rightness of action as knowledge; and this is a point we omitted just now in our consideration of the nature of virtue,