SOCRATES: That point being now determined, must we not say that he who has an opinion, must have an opinion about something which he knows or does not know?
THEAETETUS: He must.
SOCRATES: He who knows, cannot but know; and he who does not know, cannot know?
THEAETETUS: Of course.
SOCRATES: What shall we say then? When a man has a false opinion does he think that which he knows to be some other thing which he knows, and knowing both, is he at the same time ignorant of both?
THEAETETUS: That, Socrates, is impossible.
SOCRATES: But perhaps he thinks of something which he does not know as some other thing which he does not know; for example, he knows neither Theaetetus nor Socrates, and yet he fancies that Theaetetus is Socrates, or Socrates Theaetetus?
THEAETETUS: How can he?
SOCRATES: But surely he cannot suppose what he knows to be what he does not know, or what he does not know to be what he knows?
THEAETETUS: That would be monstrous.