SOCRATES: Then do we not come back to the old difficulty? For he who makes such a mistake does think one thing which he knows to be another thing which he knows; but this, as we said, was impossible, and afforded an irresistible proof of the non-existence of false opinion, because otherwise the same person would inevitably know and not know the same thing at the same time.
THEAETETUS: Most true.
SOCRATES: Then false opinion cannot be explained as a confusion of thought and sense, for in that case we could not have been mistaken about pure conceptions of thought; and thus we are obliged to say, either that false opinion does not exist, or that a man may not know that which he knows;—which alternative do you prefer?
THEAETETUS: It is hard to determine, Socrates.
SOCRATES: And yet the argument will scarcely admit of both. But, as we are at our wits' end, suppose that we do a shameless thing?
THEAETETUS: What is it?
SOCRATES: Let us attempt to explain the verb 'to know.'
THEAETETUS: And why should that be shameless?
SOCRATES: You seem not to be aware that the whole of our discussion from the very beginning has been a search after knowledge, of which we are assumed not to know the nature.
THEAETETUS: Nay, but I am well aware.