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.