This is a dialogue between Thaetetus and a philosopher visiting Athens from Elea, simply referred to throughout as the Visitor. It is a very civil conversation, with the two most often agreeing with one another and working together to move the arguments along. The dialogue’s main purpose is ostensibly to define what a sophist is. However, as the two men dig deeper, they must seek to parse out the natures of knowledge, being, belief, negation, sameness, and difference, all the while analyzing the meanings of forms and kinds.
The Visitor begins by explaining the difference between the educated and the ignorant. “Not knowing, but thinking that you know. That’s what probably causes all the mistakes we make when we think…. If someone thinks he’s wise, he’ll never be willing to learn anything about what he thinks he’s clever at…. [Therefore,] refutation is the principal and most important kind of cleansing.” The Visitor goes on to relate the sophist’s methods. “This appearing, and this seeming but not being, and this saying things but not true things—all these issues are full of confusion, just as they always have been.” He quotes his mentor Parmenides’ most famous dictum, “Never shall this force itself on us, that that which is not may be; While you search, keep your thought far away from this path.” The Visitor then asks of Thaetetus, “This form of speech of ours involves the rash assumption that that which is not is, since otherwise falsity wouldn’t come into being…. Do we dare to utter the sound that which in no way is?”
Thaetetus and the Visitor next explore this idea of negation in detail, first just scratching the surface. The Visitor begins, “That which is not can’t be applied to any of those which are…. We always apply this something to a being…. A person who says something has to be saying some one thing?… Someone who does not say something says nothing at all…. Therefore don’t we have to refuse to admit that a person like that speaks but says nothing? Instead, don’t we have to deny that anyone who tries to utter that which is not is even speaking?… But shall we say that any of those which are can ever belong to that which is not?… Whenever we speak of those which are not, aren’t we trying to apply numerical plurality to them?… Do you understand, then, that it’s impossible to say, speak, or think that which is not correctly by itself? It’s unthinkable, unsayable, unutterable, and unformulable in speech.” That is what the Visitor had learned from his mentor, Parmenides. However, the Visitor has already come to see how he contradicts his own thoughts. “I was the one who made the statement that that which is not should not share either in one or in plurality. But even so I’ve continued after all that to speak of it as one, since I say that which is not…. So in trying to attach being to it wasn’t I saying things that were contrary of what I said before?” He ends by challenging Thaetetus, “Try to say something correct about that which is not, without attaching either being, one, or numerical plurality to it.” The Visitor realizes that he will have to apply more scrutiny on his own accepted wisdoms. “In order to defend ourselves we’re going to have to subject father Parmenides’ saying to further examination, and insist by brute force both that that which is not somehow is, and then again that that which is somehow is not.”
Thaetetus and the Visitor continue by examining the natures of being and change. The Visitor begins, “Then both that which changes and also change have to be admitted as being…. The philosopher—the person who values these things the most—absolutely has to refuse to accept the claim that everything is at rest, either from defenders of the one or from friends of the many forms. In addition he has to refuse to listen to people who make that which is change in every way. He has to be like a child begging for “both,” and say that that which is—everything—is both the unchanging and that which changes.” The Visitor tackles this challenge by examining forms and kinds. “It takes expertise is dialectic to divide things by kinds and not to think that the same form is a different one or that a different form is the same…. So if a person can do that, he’ll be capable of adequately discriminating a single form spread out all through a lot of other things, each of which stands separate from the others. In addition he can discriminate forms that are different from each other but are included within a single form that’s outside them, or a single form that’s connected as a unit throughout many wholes, or many forms that are completely separate from others. That’s what it is to know how to discriminate by kinds how things can associate and how they can’t…. Some kinds will associate with each other and some won’t. Some will to a small extent and others will associate a great deal.”
Just when the two men feel that they have made some progress, they plunge back into the depths of confusion. The Visitor states, “The most important kinds we’ve just been discussing are that which is, rest, and change…. But that which is blends with both of them, since presumably both of them are…. But what in the world are the same and the different that we’ve been speaking of? Are they two kinds other than those three but necessarily always blending with them? And do we have to think of them as being five and not three? Or have what we’ve been calling the same and the different turned out, without our realizing it, to be among those three?… It’s impossible for the same and that which is to be one…. In fact, though, it turns out that whatever is different definitely has to be what it is from something that’s different…. It pervades all of them, since each of them is different from the others.”
Thaetetus and the Visitor hope to solve their difficulties by further examining differences along with the principle of negation. The Visitor, as usual, leads the way, “So it has to be possible for that which is not to be, in the case of change and also as applied to all the kinds. That’s because as applied to all of them the nature of the different makes each of them not be, by making it different from that which is. And we’re going to be right if we say that all of them are not in this same way. And on the other hand we’re also going to be right if we call them beings, because they have a share in that which is…. So as concerning each of the forms that which is is extensive, and that which is not is indefinite in quantity…. So even that which is is not, in as many applications as there are of the others, since, not being them, it is one thing, namely itself, and on the other hand it is not those others, which are an indefinite number.” The Visitor continues by parsing out that differences are not necessarily contrary in nature. “It seems that when we say that which is not, we don’t say something contrary to that which is, but only something different from it…. So we won’t agree with somebody who says that negation signifies contrary. We’ll only admit this much: when “not” and “non-” are prefixed to names that follow them, they indicate something other than the names, or rather, other than the things to which the names following the negation are applied.”
The Visitor concludes by trying to apply negation to the nature of being. He asks, “Should we say that just as the large was large, the beautiful was beautiful, the not large was not large, and the not beautiful was not beautiful, in the same way that which is not also was and is not being, and is one form among the many that are?” When Thaetetus agrees that this is so, the Visitor realizes that he has pushed the bounds of Parmenides’ grave warnings. However, the Visitor bravely goes on, “But we’ve not only shown that those which are not are. We’ve also caused what turns out to be the form of that which is not to appear. Since we showed that the nature of the different is, chopped up among all beings in relation to each other, we dared to say that that which is not really is just this, namely, each part of the nature of the different that’s set over against that which is…. Nobody can say that this that which is not, which we’ve made to appear and now dare to say is, is the contrary of that which is. We’ve said good-bye long ago to any contrary of that which is, and to whether it is or not, and also to whether or not an account can be given of it.”