Then since they must be in motion, and since absence of motion must be impossible for anything, all things are always in all kinds of motion.

Necessarily.

Then just examine this point of their doctrine. Did we not find that they say that heat or whiteness or anything you please arises in some such way as this, namely that each of these moves simultaneously with perception between the active and the passive element, and the passive becomes percipient, but not perception, and the active becomes, not a quality, but endowed with a quality? Now perhaps quality seems an extraordinary word, and you do not understand it when used with general application, so let me give particular examples. For the active element becomes neither heat nor whiteness, but hot or white, and other things in the same way; you probably remember that this was what we said earlier in our discourse, that nothing is in itself unvaryingly one, neither the active nor the passive, but from the union of the two with one another the perceptions and the perceived give birth and the latter become things endowed with some quality while the former become percipient.

I remember, of course.

Let us then pay no attention to other matters, whether they teach one thing or another; but let us attend strictly to this only, which is the object of our discussion. Let us ask them, Are all things, according to your doctrine, in motion and flux? Is that so?

Yes.

Have they then both kinds of motion which we distinguished? Are they moving in space and also undergoing alteration?

Of course; that is, if they are to be in perfect motion.

Then if they moved only in space, but did not undergo alteration, we could perhaps say what qualities belong to those moving things which are in flux, could we not?

That is right.

But since not even this remains fixed—that the thing in flux flows white, but changes, so that there is a flux of the very whiteness, and a change of color, that it may not in that way be convicted of remaining fixed, is it possible to give any name to a color, and yet to speak accurately?

How can it be possible, Socrates, or to give a name to anything else of this sort, if while we are speaking it always evades us, being, as it is, in flux?

But what shall we say of any of the perceptions, such as seeing or hearing? Does it perhaps remain fixed in the condition of seeing or hearing?

It must be impossible, if all things are in motion.

Then we must not speak of seeing more than not seeing, or of any other perception more than of non-perception, if all things are in all kinds of motion.

No, we must not.

And yet perception is knowledge, as Theaetetus and I said.

Yes, you did say that.

Then when we were asked what is knowledge? we answered no more what knowledge is than what not-knowledge is.

So it seems.

This would be a fine result of the correction of our answer, when we were so eager to show that all things are in motion, just for the purpose of making that answer prove to be correct. But this, I think, did prove to be true, that if all things are in motion, every answer to any question whatsoever is equally correct, and we may say it is thus or not thus—or, if you prefer, becomes thus, to avoid giving them fixity by using the word is.

You are right.

Except, Theodorus, that I said thus, and not thus; but we ought not even to say thus; for thus would no longer be in motion; nor, again, not thus. For there is no motion in this either; but some other expression must be supplied for those who maintain this doctrine, since now they have, according to their own hypothesis, no words, unless it be perhaps the word nohow. That might be most fitting for them, since it is indefinite.

At any rate that is the most appropriate form of speech for them.

So, Theodorus, we have got rid of your friend, and we do not yet concede to him that every man is a measure of all things, unless he be a sensible man; and we are not going to concede that knowledge is perception, at least not by the theory of universal motion, unless Theaetetus here has something different to say.

An excellent idea, Socrates; for now that this matter is settled, I too should be rid of the duty of answering your questions according to our agreement, since the argument about Protagoras is ended.

No, Theodorus, not until you and Socrates have discussed those who say all things are at rest, as you proposed just now.

A young man like you, Theaetetus, teaching your elders to do wrong by breaking their agreements! No; prepare to answer Socrates yourself for the rest of the argument.

I will if he wishes it. But I should have liked best to hear about the doctrine I mentioned.

Calling Socrates to an argument is calling cavalry into an open plain. A proverbial expression. An open plain is just what cavalry desires. Just ask him a question and you shall hear.

Still I think, Theodorus, I shall not comply with the request of Theaetetus.

Why will you not comply with it?

Because I have a reverential fear of examining in a flippant manner Melissus and the others who teach that the universe is one and motionless, and because I reverence still more one man, Parmenides. Parmenides seems to me to be, in Homer’s words, one to be venerated and also awful. Il. 3.172;Od. 8.22; xiv. 234 For I met him when I was very young and he was very old, and he appeared to me to possess an absolutely noble depth of mind. So I am afraid we may not understand his words and may be still farther from understanding what he meant by them; but my chief fear is that the question with which we started, about the nature of knowledge, may fail to be investigated, because of the disorderly crowd of arguments which will burst in upon us if we let them in; especially as the argument we are now proposing is of vast extent, and would not receive its deserts if we treated it as a side issue, and if we treat it as it deserves, it will take so long as to do away with the discussion about knowledge. Neither of these things ought to happen, but we ought to try by the science of midwifery to deliver Theaetetus of the thoughts about knowledge with which he is pregnant.

Yes, if that is your opinion, we ought to do so.

Consider, then, Theaetetus, this further point about what has been said. Now you answered that perception is knowledge, did you not?

Yes.

If, then, anyone should ask you, By what does a man see white and black colors and by what does he hear high and low tones? you would, I fancy, say, By his eyes and ears.

Yes, I should.

The easy use of words and phrases and the avoidance of strict precision is in general a sign of good breeding; indeed, the opposite is hardly worthy of a gentleman, but sometimes it is necessary, as now it is necessary to object to your answer, in so far as it is incorrect. Just consider; which answer is more correct, that our eyes are that by which we see or that through which we see, and our ears that by which or that through which we hear?

I think, Socrates, we perceive through, rather than by them, in each case.

Yes, for it would be strange indeed, my boy, if there are many senses ensconced within us, as if we were so many wooden horses of Troy, and they do not all unite in one power, whether we should call it soul or something else, by which we perceive through these as instruments the objects of perception.

I think what you suggest is more likely than the other way.

Now the reason why I am so precise about the matter is this: I want to know whether there is some one and the same power within ourselves by which we perceive black and white through the eyes, and again other qualities through the other organs, and whether you will be able, if asked, to refer all such activities to the body. But perhaps it is better that you make the statement in answer to a question than that I should take all the trouble for you. So tell me: do you not think that all the organs through which you perceive hot and hard and light and sweet are parts of the body? Or are they parts of something else?

Of nothing else.

And will you also be ready to agree that it is impossible to perceive through one sense what you perceive through another; for instance, to perceive through sight what you perceive through hearing, or through hearing what you perceive through sight?

Of course I shall.

Then if you have any thought about both of these together, you would not have perception about both together either through one organ or through the other.

No.

Now in regard to sound and color, you have, in the first place, this thought about both of them, that they both exist?

Certainly.

And that each is different from the other and the same as itself?

Of course.

And that both together are two and each separately is one?

Yes, that also.

And are you able also to observe whether they are like or unlike each other?

May be.

Now through what organ do you think all this about them? For it is impossible to grasp that which is common to them both either through hearing or through sight. Here is further evidence for the point I am trying to make: if it were possible to investigate the question whether the two, sound and color, are bitter or not, you know that you will be able to tell by what faculty you will investigate it, and that is clearly neither hearing nor sight, but something else.

Of course it is,—the faculty exerted through the tongue.

Very good. But through what organ is the faculty exerted which makes known to you that which is common to all things, as well as to these of which we are speaking—that which you call being and not-being, and the other attributes of things, about which we were asking just now? What organs will you assign for all these, through which that part of us which perceives gains perception of each and all of them?

You mean being and not-being, and likeness and unlikeness, and identity and difference, and also unity and plurality as applied to them. And you are evidently asking also through what bodily organs we perceive by our soul the odd and the even and everything else that is in the same category.

Bravo, Theaetetus! you follow me exactly; that is just what I mean by my question.

By Zeus, Socrates, I cannot answer, except that I think there is no special organ at all for these notions, as there are for those others; but it appears to me that the soul views by itself directly what all things have in common.

Why, you are beautiful, Theaetetus, and not, as Theodorus said, ugly; for he who speaks beautifully is beautiful and good. But besides being beautiful, you have done me a favor by relieving me from a long discussion, if you think that the soul views some things by itself directly and others through the bodily faculties; for that was my own opinion, and I wanted you to agree.

Well, I do think so.

To which class, then, do you assign being; for this, more than anything else, belongs to all things?

I assign them to the class of notions which the soul grasps by itself directly.

And also likeness and unlikeness and identity and difference?

Yes.

And how about beautiful and ugly, and good and bad?

I think that these also are among the things the essence of which the soul most certainly views in their relations to one another, reflecting within itself upon the past and present in relation to the future.

Stop there. Does it not perceive the hardness of the hard through touch, and likewise the softness of the soft?

Yes.

But their essential nature and the fact that they exist, and their opposition to one another, and, in turn, the essential nature of this opposition, the soul itself tries to determine for us by reverting to them and comparing them with one another.

Certainly.

Is it not true, then, that all sensations which reach the soul through the body, can be perceived by human beings, and also by animals, from the moment of birth; whereas reflections about these, with reference to their being and usefulness, are acquired, if at all, with difficulty and slowly, through many troubles, in other words, through education?

Assuredly.

Is it, then, possible for one to attain truth who cannot even get as far as being?

No.

And will a man ever have knowledge of anything the truth of which he fails to attain?

How can he, Socrates?

Then knowledge is not in the sensations, but in the process of reasoning about them; for it is possible, apparently, to apprehend being and truth by reasoning, but not by sensation.

So it seems.

Then will you call the two by the same name, when there are so great differences between them?

No, that would certainly not be right.

What name will you give, then, to the one which includes seeing, hearing, smelling, being cold, and being hot?

Perceiving. What other name can I give it?

Collectively you call it, then, perception?

Of course.

By which, we say, we are quite unable to apprehend truth, since we cannot apprehend being, either.

No; certainly not.

Nor knowledge either, then.

No.

Then, Theaetetus, perception and knowledge could never be the same.

Evidently not, Socrates; and indeed now at last it has been made perfectly clear that knowledge is something different from perception.