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.
Socrates: Where, then, is false opinion? For if all things are either known or unknown, there can be no opinion which is not comprehended under this alternative, and so false opinion is excluded.
Theaetetus: Most true.
Socrates: Suppose that we remove the question out of the sphere of knowing or not knowing, into that of being and not-being.
Theaetetus: What do you mean?
Socrates: May we not suspect the simple truth to be that he who thinks about anything, that which is not, will necessarily think what is false, whatever in other respects may be the state of his mind?
Theaetetus: That, again, is not unlikely, Socrates.
Socrates: Then suppose someone to say to us, Theaetetus:—Is it possible for any man to think that which is not, either as a self-existent substance or as a predicate of something else? And suppose that we answer, “Yes, he can, when he thinks what is not true.”—That will be our answer?
Theaetetus: Yes.
Socrates: But is there any parallel to this?
Theaetetus: What do you mean?
Socrates: Can a man see something and yet see nothing?
Theaetetus: Impossible.
Socrates: But if he sees any one thing, he sees something that exists. Do you suppose that what is one is ever to be found among non-existing things?
Theaetetus: I do not.
Socrates: He then who sees some one thing, sees something which is?