Sophist 247d-249a
V. Then let's go back to questioning them (the earth giants, the people who believe that only things that have body are) . It's enough if they admit that even a small part of that which is doesn't have body. They need to say something about what's common to both it and the things that do have body, which they focus on when they say that they both are. Maybe that will raise some confusion for them. If it does, then think about whether they'd be willing to accept our suggestion that that which is is something like the following.
T. Like what? Tell me and maybe we'll know.
V. I'm saying that a thing really is if it has any capacity at all, either by nature to do something to something else or to have even the smallest thing done to it, even the most trivial thing, even if it only happens once. I'll take it as a definition that those which are amount to nothing other than capacity.
T. They accept that, since they don't ahve anything better to say right now.
V. Fine. Maybe something else will occur to them later, and to us too. For now let's agree with them on this much.
T. All right.
Visitor: Let's turn to the friends of the forms. You serve as their interpreter for us.
Theaetetus: All right.
V: You people distinguish coming-to-be and being and say that they are separate? Is that right?
T: "Yes."
V. And you say that by our bodies and through perception we have dealings with coming-to-be, but we deal with real being by our souls and through reasoning. You say that being always stays the same and in the same state, but coming-to-be varies from one time to another.
T: "We do say that."
V. And what shall we say this dealing with is that you apply in the two cases? Doesn't it mean what we said just now?
T; "What?"
V. What happens when two things come together, and by some capacity one does something to the other or has something done to it. Or maybe you don't hear their answer clearly, Theaetetus. But I do, probably because I'm used to them.
T. Then what account do they give?
V. They don't agree with what we just said to the earth people about being.
T. What's that?
V. We took it as a sufficient definition of beings that the capacity be present in a thing to do something or have something done to it, to or by even the smallest thing or degree.
T. Yes.
V. In reply they say that coming-to-be has the capacity to do something or have something done to it, but that this capacity doesn't fit with being.
T. Is there anything to that?
V. We have to reply that we need them to tell us more clearly whether they agree that the soul knows and also that being is known.
T. "Yes," they say.
V. Well then, do you say that knowing and being known are cases of doing, or having something done, or both? Is one of them doing and the other having something done? Or is neither a case of either?
T. Obviously neither is a case of either, since otherwise they'd be saying something contrary to what they said before.
V. Oh, I see. You mean that if knowing is doing something, then necessarily what is known has something done to it. When being is known by knowledge, according to this account, then insofar as it's known it's changed by having something done to it--which we say wouldn't happen to something that's at rest.
T. That's correct.
V. But for heaven's sake, are we going to be convinced that it's true that change, life, soul, and intelligence are not present in that which wholly is, and that it neither lives nor thinks, but stays changeless, solemn, and holy, without any understanding?