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 some one 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?