SOC. Then this, at any rate, is possible for us, is it not, regarding all things collectively and each thing separately, either to know or not to know them? For learning and forgetting, as intermediate stages, I leave out of account for the present, for just now they have no bearing upon our argument.

THEAET. Certainly, Socrates, nothing is left in any particular case except knowing or not knowing it.

SOC. Then he who forms opinion must form opinion either about what he knows or about what he does not know?

THEAET. Necessarily.

SOC. And it is surely impossible that one who knows a thing does not know it, or that one who does not know it knows it.

THEAET. Certainly.

SOC. Then does he who forms false opinions think that the things which he knows are not these things, but some others of the things he knows, and so, knowing both, is he ignorant of both?

THEAET. That is impossible, Socrates.

SOC. Well then, does he think that the things he does not know are other things which he does not know—which is as if a man who knows neither Theaetetus nor Socrates should conceive the idea that Socrates is Theaetetus or Theaetetus Socrates?

THEAET. That is impossible.

SOC. But surely a man does not think that the things he knows are the things he does not know, or again that the things he does not know are the things he knows.

THEAET. That would be a monstrous absurdity.

SOC. Then how could he still form false opinions? For inasmuch as all things are either known or unknown to us, it is impossible, I imagine, to form opinions outside of these alternatives, and within them it is clear that there is no place for fake opinion.

THEAET. Very true.

SOC. Had we, then, better look for what we are seeking, not by this method of knowing and not knowing, but by that of being and not being?

THEAET. What do you mean?

SOC. We may simply assert that he who on any subject holds opinions which are not, will certainly think falsely, no matter what the condition of his mind may be in other respects.

THEAET. That, again, is likely, Socrates.

SOC. Well then, what shall we say, Theaetetus, if anyone asks us, Is that which is assumed in common speech possible at all, and can any human being hold an opinion which is not, whether it be concerned with any of the things which are, or be entirely independent of them? We, I fancy, shall reply, Yes, when, in thinking, he thinks what is not true, shall we not?

THEAET. Yes.

SOC. And is the same sort of thing possible in any other field?

THEAET. What sort of thing?

SOC. For instance, that a man sees something, but sees nothing.

THEAET. How can he?

SOC. Yet surely if a man sees any one thing, he sees something that is. Or do you, perhaps, think one is among the things that are not?

THEAET. No, I do not.

SOC. Then he who sees any one thing, sees something that is.

THEAET. That is clear.