But surely we did not begin our conversation in order to find out what knowledge is not, but what it is. However, we have progressed so far, at least, as not to seek for knowledge in perception at all, but in some function of the soul, whatever name is given to it when it alone and by itself is engaged directly with realities.

That, Socrates, is, I suppose, called having opinion.

You suppose rightly, my friend. Now begin again at the beginning. Wipe out all we said before, and see if you have any clearer vision, now that you have advanced to this point. Say once more what knowledge is.

To say that all opinion is knowledge is impossible, Socrates, for there is also false opinion; but true opinion probably is knowledge. Let that be my answer. For if it is proved to be wrong as we proceed, I will try to give another, just as I have given this.

That is the right way, Theaetetus. It is better to speak up boldly than to hesitate about answering, as you did at first. For if we act in this way, one of two things will happen: either we shall find what we are after, or we shall be less inclined to think we know what we do not know at all; and surely even that would be a recompense not to be despised. Well, then, what do you say now? Assuming that there are two kinds of opinion, one true and the other false, do you define knowledge as the true opinion?

Yes. That now seems to me to be correct.

Is it, then, still worth while, in regard to opinion, to take up again—?

What point do you refer to?

Somehow I am troubled now and have often been troubled before, so that I have been much perplexed in my own reflections and in talking with others, because I cannot tell what this experience is which we human beings have, and how it comes about.

What experience?

That anyone has false opinions. And so I am considering and am still in doubt whether we had better let it go or examine it by another method than the one we followed a while ago.

Why not, Socrates, if there seems to be the least need of it? For just now, in talking about leisure, you and Theodorus said very truly that there is no hurry in discussions of this sort.

You are right in reminding me. For perhaps this is a good time to retrace our steps. For it is better to finish a little task well than a great deal imperfectly.

Of course.

How, then, shall we set about it? What is it that we do say? Do we say that in every case of opinion there is a false opinion, and one of us has a false, and another a true opinion, because, as we believe, it is in the nature of things that this should be so?

Yes, we do.

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.

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

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

Necessarily.

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.

Certainly.

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?

That is impossible, Socrates.

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?

That is impossible.

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.

That would be a monstrous absurdity.

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.

Very true.

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?

What do you mean?

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.

That, again, is likely, Socrates.

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?

Yes.

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

What sort of thing?

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

How can he?

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?

No, I do not.

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

That is clear.

And therefore he who hears anything, hears some one thing and therefore hears what is.

Yes.

And he who touches anything, touches some one thing, which is, since it is one?

That also is true.

So, then, does not he who holds an opinion hold an opinion of some one thing?

He must do so.

And does not he who holds an opinion of some one thing hold an opinion of something that is?

I agree.

Then he who holds an opinion of what is not holds an opinion of nothing.

Evidently.

Well then, he who holds an opinion of nothing, holds no opinion at all.

That is plain, apparently.

Then it is impossible to hold an opinion of that which is not, either in relation to things that are, or independently of them.

Evidently.

Then holding false opinion is something different from holding an opinion of that which is not?

So it seems.

Then false opinion is not found to exist in us either by this method or by that which we followed a little while ago.

No, it certainly is not.

But does not that which we call by that name arise after the following manner?

After what manner?

We say that false opinion is a kind of interchanged opinion, when a person makes an exchange in his mind and says that one thing which exists is another thing which exists. For in this way he always holds an opinion of what exists, but of one thing instead of another; so he misses the object he was aiming at in his thought and might fairly be said to hold a false opinion.

Now you seem to me to have said what is perfectly right. For when a man, in forming an opinion, puts ugly instead of beautiful, or beautiful instead of ugly, he does truly hold a false opinion.

Evidently, Theaetetus, you feel contempt of me, and not fear.

Why in the world do you say that?

You think, I fancy, that I would not attack your truly false by asking whether it is possible for a thing to become slowly quick or heavily light, or any other opposite, by a process opposite to itself, in accordance, not with its own nature, but with that of its opposite. But I let this pass, that your courage may not fail. You are satisfied, you say, that false opinion is interchanged opinion?

I am.

It is, then, in your opinion, possible for the mind to regard one thing as another and not as what it is.

Yes, it is.

Now when one’s mind does this, does it not necessarily have a thought either of both things together or of one or the other of them?

Yes, it must; either of both at the same time or in succession.

Excellent. And do you define thought as I do?

How do you define it?

As the talk which the soul has with itself about any subjects which it considers. You must not suppose that I know this that I am declaring to you. But the soul, as the image presents itself to me, when it thinks, is merely conversing with itself, asking itself questions and answering, affirming and denying. When it has arrived at a decision, whether slowly or with a sudden bound, and is at last agreed, and is not in doubt, we call that its opinion; and so I define forming opinion as talking and opinion as talk which has been held, not with someone else, nor yet aloud, but in silence with oneself. How do you define it?

In the same way.

Then whenever a man has an opinion that one thing is another, he says to himself, we believe, that the one thing is the other.

Certainly.

Now call to mind whether you have ever said to yourself that the beautiful is most assuredly ugly, or the wrong right, or—and this is the sum of the whole matter—consider whether you have ever tried to persuade yourself that one thing is most assuredly another, or whether quite the contrary is the case, and you have never ventured, even in sleep, to say to yourself that the odd is, after all, certainly even, or anything of that sort.

You are right.

Do you imagine that anyone else, sane or insane, ever ventured to say to himself seriously and try to persuade himself that the ox must necessarily be a horse, or two one?

No, by Zeus, I do not.

Then if forming opinion is talking to oneself, no one who talks and forms opinion of two objects and apprehends them both with his soul, could say and have the opinion that one is the other. But you will also have to give up the expression one and other. This is what I mean, that nobody holds the opinion that the ugly is beautiful, or anything of that sort.

Well, Socrates, I do give it up; and I agree with you in what you say.

You agree, therefore, that he who holds an opinion of both things cannot hold the opinion that one is the other.

So it seems.

But surely he who holds an opinion of one only, and not of the other at all, will never hold the opinion that one is the other.

You are right; for he would be forced to apprehend also that of which he holds no opinion.

Then neither he who holds opinion of both nor he who holds it of one can hold the opinion that a thing is something else. And so anyone who sets out to define false opinion as interchanged opinion would be talking nonsense. Then neither by this method nor by our previous methods is false opinion found to exist in us.

Apparently not.

But yet, Theaetetus, if this is found not to exist, we shall be forced to admit many absurdities.

What absurdities?