SOCRATES: If someone knows the way to Larissa, or where you will, and goes there and guides others, will he not guide rightly and well? Well, what of one who has never been there, and does not know the way; but if the he has a right opinion as to the way, won't he also guide rightly? And as so long as he has right opinion about that of which the other has knowledge, he will be quite as good a guide as the one who knows, although he does not know, but only thinks, what is true. Then, true opinion is no worse guide than wisdom, for rightness of action [...]. We said then that wisdom alone guides right action; but really, true opinion does the same. Then right opinion is no less useful than knowledge.
MENON: Yes, it is less useful; for he who has knowledge would always be right, he who has right opinion, only sometimes.
SOCRATES: What! Would not he that has right opinion always be right as long as he had right opinion?
MENON: Oh, yes, necessarily, I think. This being so, I am surprised, Socrates, why knowledge is ever more valued than right opinion, and why they are two different things.
SOCRATES: For the true opinions, as long as they stay, are splendid and do all the good in the world; but they will not stay long -- off and away they run out of the soul of mankind, so they are not worth much until you fasten them up with reasoning of cause and effect. [...] When they are fastened up, first they become knowledge, secondly, they remain; and that is why knowledge is valued more than right opinion, and differs from right opinion by this bond. Well, I speak by conjecture, not as one who knows; but to say that right opinion is different from knowledge, that, I believe, is no conjecture in me at all. That I would say I know; there are few things I would say that of, but this I would certainly put down as one of those I know.
Opinion in good man is knowledge in the making. -- Milton, Areopagitica.