SOCRATES: How can the exchange of one knowledge for another ever become false opinion?
THEAETETUS: What do you mean?
SOCRATES: In the first place, how can a man who has the knowledge of anything be ignorant of that which he knows, not by reason of ignorance, but by reason of his own knowledge? And, again, is it not an extreme absurdity that he should suppose another thing to be this, and this to be another thing;—that, having knowledge present with him in his mind, he should still know nothing and be ignorant of all things?—you might as well argue that ignorance may make a man know, and blindness make him see, as that knowledge can make him ignorant.
THEAETETUS: Perhaps, Socrates, we may have been wrong in making only forms of knowledge our birds: whereas there ought to have been forms of ignorance as well, flying about together in the mind, and then he who sought to take one of them might sometimes catch a form of knowledge, and sometimes a form of ignorance; and thus he would have a false opinion from ignorance, but a true one from knowledge, about the same thing.
SOCRATES: I cannot help praising you, Theaetetus, and yet I must beg you to reconsider your words. Let us grant what you say—then, according to you, he who takes ignorance will have a false opinion—am I right?
THEAETETUS: Yes.
SOCRATES: He will certainly not think that he has a false opinion?
THEAETETUS: Of course not.
SOCRATES: He will think that his opinion is true, and he will fancy that he knows the things about which he has been deceived?
THEAETETUS: Certainly.