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Plato's Theaetetus - Selections - Comments

Tr. F.M. Cornford. This dialog was also known in classical times by the alternative title "On Knowledge" (Diog. L. iii, 58).

What is knowledge? In this dialog knowledge is presumed to be a single Form (Idea), everywhere the same, whereas in the Philebus it is argued that knowledge is many different things.


"What is the essence of knowledge, the one defining thing that all knowledge has in common, its common nature?" Plato asks in the Theaetetus (146e), his assumption being that knowledge is one thing, always that same. But in a later dialog he will ask, "How can knowledge be one thing, but at the same time there be many branches of knowledge, some of which are unalike or even opposite to one another?" (Philebus 13e-14a).

In 1944 Wittgenstein wrote to Drury that he was reading Plato's Theaetetus --

"Plato in this dialogue is occupied with the same problems that I am writing about." A little later he sent [Drury] a copy of a translation of the Theaetetus ... I had to write back to him saying that I found it "cold". His reply was, "It was far from cold when it was written." (Recollections p. 149)

Wittgenstein refers to the Theaetetus in The Blue Book, which dates from 1933-1934). What were "the same problems" Wittgenstein alluded to? Maybe see Theaetetus 147a-c.

My remarks on this page have as their background the study of logic of language (Wittgenstein's expression, but as my jargon), which identifies the first question in philosophy as: How is an objective distinction made between sense and nonsense -- i.e. between defined language and undefined combinations of words -- in philosophy?

But I have also taken note of some things outside logic-philosophy, for there are three parts to Philosophy: logic, metaphysics, ethics (if we want to slice the pie -- i.e. the whole that is Philosophy into parts -- the way the early Stoics did) as all three concerned Plato and as surely they should also concern us.

It is in this dialog that Plato introduces the thesis that "knowledge is justified true belief", and on this page that thesis is criticized for misclassifying knowledge as a type of belief, as if 'knowledge' and 'belief' were not distinct and independent concepts. Further, it is necessary to cast out any nebulous mists of psychology Plato's introduction of the word 'belief' may conjure up. Philosophy must demystify our concepts, not nurture mystification by means of them.

In English 'wisdom' and 'knowledge' are different concepts (i.e. we use those words differently), and yet both in Xenophon's account and in the following account of Plato's, the words 'wisdom' and 'knowledge' are said to name the same thing. So that from the beginning we find that we are not quite clear about what Plato is talking about.

Outline of this page ...

What is the common nature of everything we call 'knowledge'?

145e-146a - SOCRATES: Then knowledge and wisdom are the same thing?


SOCRATES: Well, that is precisely what I am puzzled about. I cannot make out to my own satisfaction what knowledge is. Can we answer that question?

Plato's assumption here is that knowledge is one thing (or, that 'knowledge' is the name of a single Form), that there is an essence of knowledge, rather than a variety of things that we call 'knowledge'. We can describe the various ways we use the word 'know', but is there a defining common nature to everything we call 'knowledge'? Being perplexed, puzzled -- because if knowledge is a single Form, then it is a unity; it can't be a multiplicity: It is just one thing, always the same thing. (But is this an example of word magic, for why should the word 'knowledge' be taken as proof that there is a Form that corresponds to that word, the single meaning, the only meaning, of that word? Plato asks similar questions in the Philebus.)

146e - But the question you were asked, Theaetetus, was not, what are the objects of knowledge, nor yet how many sorts of knowledge there are. We did not want to count them, but to find out what the thing itself -- knowledge -- is.

A general definition of clay. | What is the essence of clay?

Question: which is logically prior to the other -- the concept 'clay' or the essence of the various phenomena we call 'clay'? Do concepts define phenomena or do phenomena define concepts (Are percepts without concepts blind)? Is sand not a kind of earth? We do not call sand mixed with moisture clay.

As an example of a general definition, in the Laches Plato gives "quickness", which he defines as "the quality which accomplishes much in a little time" (192a), regardless of what particular thing is quick.

An example we might give is our common definition of the English word 'simile' which is: a comparison using the word 'as' or 'like'. For anything to be a 'simile' it must (1) be a comparison, and (2) use the word 'as' or 'like'. Those two requirements not only state the defining common-quality of all similes, but they also distinguish similes from all other classes [categories, concepts] of things, and therefore this is an example of a Socratic definition. (Contrast the way Aristotle describes Socrates' method as inductive -- i.e. it begins by gathering examples -- with the method Plato uses in this dialog, for they are not the same method.)

Socrates as a barren midwife

150b-c - I am so far like the midwife that I cannot myself give birth to wisdom, and the common reproach is true, that, though I question others, I can myself bring nothing to light because there is no wisdom in me.... And the highest point of my art is the power to prove by every test whether the offspring of a young man's thought is a false phantom or instinct with life and truth.

151e - ... now suppose we examine your offspring together, and see whether it is a mere wind egg or has some life in it.

That is the Socrates of Plato, the more or less literary invention (or inventions, for the character of "Socrates" varies from one dialog to the next) of Plato. But the Socrates of Xenophon "never gave up considering with his companions what any given thing is. To go through all his definitions would be an arduous task" (iv, 6, 1). But he did have definitions, things that he himself had brought to light.

148d - ... set your mind on finding a definition of knowledge ...

What is knowledge according to Protagoras?

151e-152e - ... so far as I can see at present, knowledge is nothing but perception. [163d-e]

SOCRATES: The account you give of the nature of knowledge is not, by any means, to be despised. It was the same that was given by Protagoras ... He says, you will remember, that "man is the measure of all things -- alike of the being of things that are and of the not-being of things that are not".... He puts it in this sort of way, doesn't he, that any given thing "is to me such as it appears to me, and is to you such as it appears to you", you and I being men? [160c-d]

152c - 'Appearing', then, is the same thing as 'perceiving' ... Perception, then, is always of something that is, and, as being knowledge, it is infallible.

Socrates held that if anyone knew something he could give an account of it to others. A concise criticism of Protagoras's view is given in Cratylus 386a ff.

152d-e - ... the doctrine is a remarkable one.... We are wrong [it says] to speak of [anything] as "being", for none of them ever is; they are always becoming [something else]. In this matter let us take it that, with the exception of Parmenides, the whole series of philosophers agree -- Protagoras, Heraclitus, Empedocles -- and among the poets the greatest masters ... When Homer speaks of "Oceanus, source of the gods, and mother Tethys" [Iliad 14] he means that all things are the offspring of a flowing stream of change ["... the doctrine of Homer and Heraclitus and all their tribe that all things move like flowing streams" (160d)].

Heraclitus' doctrine that everything is in flux is stated in Cratylus 401d and 402a. And it is the foundation of Plato's thinking in Metaphysics (for not only Socrates, but also Heraclitus is Plato's teacher in philosophy) that what man perceives is in a state of constant change -- but that knowledge can only be of what is unchanging (permanent, eternal), which is what Plato's supersensible (and existing independently of the perceptible things which "partake" of them) Forms ("Patterns", "Archetypes", "Ideals" or "Absolutes") are imagined by him to be.

When Protagoras says, that "Man is the measure of all things, of what is that it is, and of what is not that it is not" [160c-d], his view agrees with Heraclitus' view [160d]. But Plato's view is that, despite Heraclitus' doctrine, knowledge must be possible. Now, as we normally use the word 'knowledge', knowledge is objective, not subjective -- i.e. it is not relative to the individual. And so Protagoras is either saying that knowledge is impossible or he is redefining the word 'knowledge', but for Plato that would amount to the same thing. Plato uses the word 'knowledge' as we normally do in this respect: that knowledge is objective; Plato's Forms are objective. But Plato's Forms are metaphysical constructs (theory), not logic of language (i.e. a description of how we normally use the word 'knowledge).

Metaphysics and Plato ("The reality behind reality")

For Plato ... anyone who spoke about [the phenomenal world] without going beyond it for its ground and explanation [as was "the Eleatic tradition, which not only found true reality outside phenomena but refused the claim that the phenomenal world was real at all", which the Sophists revolted against when "they rejected just what [Plato] regarded as of the highest value and reality (see e.g. Phaedo 101e)"] could not possibly be seeking the truth.... But in the modern world ... the majority of scholars are not Platonists and, for the most part, do not even wish to look for reality in the direction where Plato believed it lay ... The time is long past when the rejection of any transcendent reality can be taken as evidence that the search for truth has been abandoned. (G.B. Kerferd, "Sophists" in Encyclopedia of Philosophy (1967), vol. 7, p. 495)

Well, there is a lot to be said here. First, rather than "going beyond" phenomena it may be clearer to say "going behind" them (for that is the picture: the truth is in the background: all we see are reflections or shadows of it). Second, the expression 'true reality' is not helpful; 'reality' does not need a modifier; 'true' contrasts with 'false', but we do describe a 'false reality' with false statements. (Sophist 257b) Third, Plato himself rejected the second Eleatic clause: instead, both rest and motion are real (ibid. 249c-d); without rest [permanence], there cannot be forms [a "fluctuating form" would be no form (Theaetetus 182d)], and without forms there can be no knowledge [For Plato "only the definable could be known" (Guthrie) [cf. Theaetetus 201c-d], and for Plato a definition is the identification of a form], discourse (ibid. 259e-260a) or therefore philosophy (ibid. 260a-b). [Aristotle says that Plato always remained faithful to this part of the Heraclitean doctrine -- that "all things perceived by the senses are in incessant flux, and there is no such thing as scientific knowledge of them" (Metaphysics 987a, tr. Taylor). What our senses perceive -- or appear to perceive -- is constantly changing, and therefore if we find it intelligible it is not through the eyes of senses that we see it but through the eye of the intellect: it is the intellect or soul that perceives Forms in the what is otherwise unintelligible flux.]

If Kerferd is correct, then in Plato's view the Sophists would have to have been strict followers of Heraclitus: there is no rest, only motion. Even if the Sophists were concerned with natural philosophy ("physics"), it was certainly not modern science they had in mind but a rival metaphysics.

"... rejection of any transcendent [supra-sensual] reality ... the search for truth has been abandoned" -- Well, I don't know about that; I really don't know. Clearly if the world has anything like a philosophical explanation, that explanation is not found in this world; the visible world does not explain itself: it does not explain why it exists rather than nothing. The abandonment of metaphysics in favor of natural philosophy -- i.e. the natural sciences -- is that not the abandonment of philosophy? It would not be unfair to say that Wittgenstein abandoned philosophy; he called his own work in philosophy -- i.e. what I call 'logic' -- no more than one of historical philosophy's heirs (BB p. 28). Now, obviously, the reign of his anti-philosophical [anti-metaphysical] point of view is not going to be eternal.

The various pictures of the world, of the real, invented by the pre-Socratics -- these are not nonsense; they are metaphysics. The trouble with these pictures is that they cannot be compared with experience. And if we want to say something about reality, as opposed to simply saying how we might picture what is behind our perceptions of it, then the issue of verification -- of the confrontation of idea [i.e. the pictures we invent] with experience -- cannot be avoided.

What is the point of their speculation? They draw all these pictures. Well, if you like you can put them in a picture book. But with respect to knowing reality they get you no further.

In Plato philosophy -- not in Plato's philosophy according to Plato -- it is as if the world cast a shadow, and that shadow were reality. The shadow is more real than what casts it. [Shadow: form of tree; what casts it: actual trees.] The shadow reality of Plato. The forms belong to the shadow reality. The shadows are more real than what casts them. [This stands Plato on his head, because what he claims is that experience, phenomena, are the "real" shadows; what casts them is supra-experiential.] The forms are more real than what "partakes" of them.

If I were to say 'I have never been to the moon -- but I may be mistaken', that would be idiotic.

For even the thought that I might have been transported there, by unknown means, in my sleep, would not give me any right to speak of a possible mistake here. I play the game wrong if I do. (OC § 662)

But why "idiotic"? -- In the context of language, the only "rights" are grammatical rights. Then would it be a senseless/meaningless utterance? Or is this "nuance"? No, it is our eternal "pictures". We distinguish between empirical possibility and fantasy: saying 'I may be mistaken' about a moon visit would be like saying 'I may be mistaken' about the existence of elves. Where verification is impossible (logically impossible, i.e. "impossible, by definition" because no possibility has been defined, no criteria stated), it makes no sense to say 'I may be mistaken'. 'I don't believe in God/in an afterlife, but I may be mistaken' -- as if these pictures were hypotheses. But if you speak of them as if they were hypotheses, "you play the game wrong" -- just as you do if you ask what color the number 3 is. I.e. 'you play the game wrong' means: your speech only expresses a confusion -- i.e. a grammatical confusion. (Of course, as always, it is the combination of words in the context in which they are uttered that is nonsense, not the individual words considered as words of the English language nor the combination of words spoken in other contexts. This is what misleads us when we try to explain what logic-of-language means by 'nonsense': it is not the meaning that is meaningless.) For Wittgenstein, is metaphysics playing the game wrong? No, fantasy pictures are not meaningless -- but, yes, treating fantasy pictures as though they were hypotheses is. Of course, the question is if "you play the game wrong", do you play the game at all? Well, you play a game [with such fantasy pictures], but not our normal one; well, but does that matter? yes, because it is not your intention to play a different game: you imagine that you are playing our normal game. Again, your statement expresses confusion -- confusion about the logic of our language, that is.

"You play the game wrong" -- you make a move in the game that is not allowed, i.e. that is nonsense -- or what meaning can your opponent attach to it: what is his next move to be? Suppose in chess you moved your pawn backward or along an angle ... but in such a case you could say that you were playing -- i.e. had invented -- a different game. But with respect to 'but I may be mistaken', the comparison/metaphor breaks down: you cannot add 'but I may be mistaken' to 'I have never been to the moon' and call that a new game: for what are the rules of this game. Suppose someone added 'but I may be mistaken' to every assertion he made; in that case, would it be that this addition was meaningless -- i.e. that 'being mistaken' was now without meaning? There is a game played according to fixed rules with 'but I may be mistaken'; but unlike chess, what changes to the rules would result in merely a new game rather than in nonsense?

Or would you be prepared to maintain that every color appears to a dog [171c] or any other creature just such as it appears to you?... Or to another man? Does anything you please appear to him such as it appears to you?

If knowledge is perception, then the dog has knowledge ... but has the dog a concept that makes his percept intelligible to him? And a wasp -- does a wasp have knowledge?

'Knowledge'. The concept is there -- like our life. We ourselves did not invent it. I myself did not invent it, but what am I to make of it? It belongs to our common currency. If it were the name of an object, as is the word 'cow', I could look at the cow [examine the cows in the field]. But it isn't. It's not the name of anything. Therefore, what is it? This is the gap in my thinking that I refuses to stay closed; so I have to keep taking this topic up again and again: what is the meaning of a word when that word is not a name? I understand ["meaning is use"], but then again I don't understand [because I am not a rest (as I am about the concept 'mind' e.g.); the nonsense question arises again and again: "What is it?"] -- i.e. I don't feel comfortable. The word 'justice' is not the name of an object, is not the name of anything. Nonetheless, the concept is there -- like our life. (OC § 559) I inherited it, just as I was born into this world. How do I determine whether justice is real -- how am I to determine what the meaning of that word is, if it has a meaning? The word has a role in our life, and that is what it means to say that 'justice exists'.

'Knowledge'. The sign [bare word] is there; -- what gives it meaning?

Philosophy begins in wonder

155c-d - SOCRATES: ... I fancy, at any rate, such puzzles are not altogether strange [i.e. unknown] to you.

THEAETETUS: No, indeed it is extraordinary how they set me wondering whatever they can mean. Sometimes I get quite dizzy with thinking of them.

SOCRATES: That shows that Theodorus was not wrong in his estimate of your nature. This sense of wonder is the mark of the philosopher. Philosophy indeed has no other origin, and he was a good genealogist who made Iris the daughter of Thaumas.

The etymology of 'Thaumas' is 'wonder', and his child named 'Iris' here appears to me to mean 'messenger of the gods' (as in: it is through wonder that we acquire wisdom, if we acquire it, for the gods alone are wise [Phaedrus 278d]), although it also might mean 'rainbow'.

[In Jowett's translation. SOCRATES: I believe that you follow me, Theaetetus; for I suspect that you have thought of these questions before now.
THEAETETUS: Yes, Socrates, and I am amazed when I think of them; by the Gods I am! and I want to know what on earth they mean; and there are times when my head quite swims with the contemplation of them.
SOCRATES: I see, my dear Theaetetus, that Theodorus had a true insight into your nature when he said that you were a philosopher; for wonder is the feeling of a philosopher, and philosophy begins in wonder.]

"Philosophy begins in wonder" looks very like G.E. Moore's testimony in his "Autobiography" that for him philosophy began in his being puzzled by the strange things philosophers said. Does 'wonder' here in Plato (155d) mean 'perplexity', 'puzzlement' at the things philosophers say? I think it includes that (151e-152a), but also what we ourselves are inclined to say -- or wonder at, e.g. right and wrong (good and evil), and natural phenomena such death and rainbows, wanting to know, "to reason out the How and Why" (The Frogs 971). What Plato would not have said was the philosophy always begins and ends in our being bewitched by language (PI § 109), although he was well aware that language mystifies us -- as does everything else in our experience, once we think about it.

In what type of case might we say that "perception is knowledge"? If you ask someone how he knows that it is raining and he replies "Because I am looking out the window". Would we say that of a dog -- a beast "wanting discourse of reason" -- that it knows that it is raining? Because N.R. Hanson would not allow that "perception is knowledge", because perceptions are concept laden (or as Hanson would say, "theory laden").

158a-e - THEAETETUS: Really, I cannot undertake to deny that madmen and dreamers believe what is false, when madmen imagine that they are gods or dreamers think they have wings and are flying in their sleep.

SOCRATES: Have you not taken note of another doubt that is raised in these cases, especially about sleeping and waking?... The question ... what evidence could be appealed to, supposing we were asked at this very moment whether we were asleep or awake, dreaming all that passes through our minds or talking to one another in the waking state?

THEAETETUS: Indeed, Socrates, I do not see by what evidence it is to be proved, for the two conditions correspond in every circumstance like exact counterparts.

SOCRATES: ... in each condition our mind strenuously contends that the convictions of the moment are certainly true, so that for equal times we affirm the reality of the one world and of the other, and are just as confident of both.... Have you any other [That the truth "is to be decided by length or shortness of time" spent in either state is rejected as "absurd in many ways"] certain test to show which of these beliefs is true?

Does the combination of words 'test whether one is awake or dreaming' have any meaning? And how is that question different from the statement that there is no such test? Does this make sense -- "Dreamers believe that such-and-such is the case"? No; 'believe' and 'disbelieve' on the basis of a test are only applied to someone who is awake. One cannot ask, "But are you awake, or only dreaming that you are looking out the window and seeing rain?" That simply isn't a move in this language-game. So, then, you are saying that that's all there are -- language-games? Well, what else would there be? [What would you prefer there to be -- philosophical theories about "what is really real"?] We use language to talk, and so we have to talk about language (PI § 120), about its meaning or how to distinguish between sense and nonsense.

160c-d - ... and I am, as Protagoras says, a judge of what is for me, that it is, and of what is not that it is not.... the doctrine of Protagoras ... that man is the measure of all things ... [152a]

161b - [Socrates says that Theodorus takes] me for a sort of bag full of arguments, and [imagines] I can easily pull out a proof ... You don't see what is happening. The arguments never come out of me; they always come from the person I am talking with. [150b] I am only at a slight advantage in having the skill to get some account of the matter [151e] from another's wisdom an entertain it with fair treatment. So now, I shall not give any explanation myself, but try to get it out of our friend.

162d-e - Protagoras or his representative will have an answer to this.... "You drag in the gods, whose existence or nonexistence I expressly refuse to discuss in my speeches and writings ... You go entirely by what looks probable, without a word of argument or proof. If a mathematician ... elected to argue from probability in geometry ... [Consider] whether you are going to allow questions of this importance to be settled by plausible appeals to mere likelihood."

Statements of truth versus plausible remarks

What is the use of studying philosophy if all that it does for you is to enable you to talk with some plausibility about some abstruse questions of logic ... (Malcolm, Memoir 2e, Letter No. 9)

Is that all we do in philosophy -- make more or less plausible remarks?

What do we mean by 'plausible' in philosophy? What we are inclined or disinclined to take seriously rather than dismiss as foolishness? Or do we mean "This is a worthwhile way to look at the thing"? If we can refute a proposition we don't call it merely "implausible", but false (or, mistaken). What are plausible in philosophy are ways of looking at things that we are inclined to accept. A philosophy is a rational way of looking at things, a point of view; within a way of looking at things, there is truth or falsity; but a way of looking at things is not itself true or false. Now, I don't think Plato would accept that account of what philosophy is.

Mere likelihood (Plausibility)

In Theaetetus 162e-163a (tr. Jowett) Plato may again (see Euthyphro 6d-7d) set mathematics up as a standard, that is to say, as a model, for testing philosophical theses against in dialectic -- but the dialog is here unclear to me: does "Protagoras, or some one speaking on his behalf" in this particular instance also speak for Plato? I don't know. Plato has that "any ... mathematician who argued from probabilities and likelihoods in geometry, would not be worth an ace". And is this not also Plato's own view as well, that "popular arguments" are commonly accepted although "not one word of proof or demonstration" is offered for them? [As to the word 'ace' which is used in both Cornford's and Jowett's translations: when counting whole numbers, the ace -- i.e. one -- is the lowest of all values.]

One cannot use plausibility as the standard for judging the truth or falsity of a theorem in geometry. But can you use that as your standard in philosophy, then? I certainly would not say that you could. (As to the meaning of Wittgenstein's letter to Malcolm (16 November 1944) in which he writes "if all [studying philosophy] does for you is to enable you to talk with some plausibility", I don't know what he meant by "some plausibility", e.g. whether meant that is all we do in philosophy.)

But question: in these pages do I myself ever use plausibility as my philosophical standard? Do I not in all instances recognize when the issue at hand is a question about ways of looking at things rather than a question about truth or falsity? I think, the statement "I don't find it implausible" isn't a weak argument -- it's no argument at all! (Of course such questions are not directed to myself alone, but also to the reader's own thinking.)

"Is sense perception the same as knowledge?"

In Theaetetus 163b (tr. Jowett), Plato has Socrates ask "whether perception is or is not the same as knowledge.... Shall we say that we know every thing which we see and hear? for example, shall we say that not having learned [the meaning of their language], we do not hear the language of foreigners when they speak to us? or shall we say that we not only hear, but know what they are saying?"

Plato here distinguishes between a sound and the sense (or, meaning) of the sound, which is the distinction the Stoics will make with their concept lekton = "what the Greek understands but barbarians do not, although they hear the sound, when Greek is spoken".

And, at least in this case, in this example, it does seem that by 'knowledge' we do not mean sense perception alone. However, do we ever mean that by 'knowledge'? Theaetetus (in 163b-c) says that: "we hear and at the same time know the rising and falling accents of the voice." But "rising and falling accents" is not as it were an innate idea: it is a concept we must learn -- i.e. we do not know "rising and falling accents" by perception alone (There is the background of knowing what language is, a background that distinguishes man from the beasts "wanting discourse of speech" -- i.e. lacking language -- as do dogs, for example. That a dog responds to tone of voice, we observe, but that a dog has our concept 'language' we do not).

Plato seeks in the Theaetetus to know the essence of knowledge, although in Philebus 13e-14a he recognizes that this may be impossible for man to know "so long as he keeps to the body" (Phaedo 66c-d) -- if, that is, Plato keeps to his own theory of Forms, which I don't know if he does -- "for how can knowledge be one thing, but at the same time there be many branches of knowledge, some of which are unalike or even opposite to one another?"

But also, as to the example Plato offers, may there not also be examples of particular cases where perception = knowledge? Are there a normal cases of language usage where we would say that perception = knowledge?

The distinction Kant makes with the concepts 'percept' and 'concept': he says that percepts without concepts are blind. His statement is a rule of grammar, because: is there anything we would call a "bare percept" -- i.e. a percept for which there is no concept but which is not without meaning? (The question is rhetorical; rules of grammar are tautologies: they are "true" "by definition".) For man, the only creature whose soul we are intimate with (cf. PI § 357), 'sound' (with meaning) versus 'noise' (without meaning) are inseparable concepts. But would we say that sounds are "without meaning" for animals (creatures without language)? Of course this asks for a definition of 'meaning' in such cases (It is not Wittgenstein's meaning of 'meaning' or "grammar" here).

We try to make analogies from man to various animals: when man behaves this way it is because ... and therefore ... But analogies do not yield therefores (To adapt Drury's statement about theories: a comparison can never become a fact; a comparison remains a comparison to all eternity.) Can there be thoughts ("meanings") without language? A girl washing dishes, a carpenter in his shop -- are there not cases where we think without using words? But there is a conceptual background to all such deeds: 'washing up', 'wood-working'. Would we ever say that to perceive is to know? (We are tangled up in questions about language here whichever way we turn. [And I am, which shames me to admit, only splashing about on the surface here, not philosophizing.]) To talk about thinking is to talk about man. To talk about knowing is to talk about man. "The infant knows its mother's face" is an analogy between a languageless creature and a creature with language. And if by 'concepts' we do not mean language ('rules for using signs'), then what shall we mean by the word 'concept'?

Variation. We hear the sound, which is a percept -- but that we hear "sounds" and that those sounds are "the sounds of a language" are concepts, and not instead percepts without meaning ("noise"). A bird's behavior shows man that a bird hears, but not that a bird hears "sounds" (The whole conceptual background for hearing "sounds" is lacking). Does a dog hear as it were a foreign language when human beings speak -- or does it only hear sound without sense, as if it were birdsong (if that's what birdsong is)? (Here maybe a distinction between an instinct, which is akin to a sensation, and a thought, which is a use of language or akin to one, could be made. But then what of wordless thoughts (e.g. When I dust the table, I must mind what I am doing, but I need not use language to direct my mindfulness)? It is not easy to see how we can define 'concepts' in cases where language is not used -- without using "but can be put into words" as our criterion.)

To return to Plato's dialog and Cornford's translation now:

163b - Well, are we going to agree that, whenever we perceive something by sight or hearing, we also at the same time know it? Take the case of a foreign language we have not learned. Are we to say that we do not hear the sounds that foreigners utter, or that we both hear and know what they are saying?

One method in philosophy is to offer counter-examples to refute a general statement (as Plato does in 163b). The distinction Plato makes here will also be made by the Stoics with their distinction between a sign and a sign's meaning (lekton). It is essential to logic's being able to make a distinction between sense and nonsense.

164c - It looks as if we were content to have reached an agreement based on mere verbal consistency ...

168b-c - ... honestly consider what we mean when we say that all things are in motion and that what seems also is, to any individual or community. The further question whether knowledge is, or is not, the same thing as perception, you will consider as a consequence of these principles, not, as you did just now, basing your argument on the common use of words and phrases [cf. 177e], which the vulgar twist into any sense they please and so perplex one another in all sorts of ways.

The good or companionable dialectician

167d-168a - Do not conduct your questioning unfairly.... one should help out the other party and bring home to him only those slips and fallacies that are due to himself or to his earlier instructors. If you follow this rule, your associates will lay the blame for their confusions and perplexities on themselves ... and disgusted with themselves, will turn to philosophy, hoping to escape from their former selves and become different men.

That is what a "good dialectician" -- (or "companionable dialectician" (Protagoras 329b and Protagoras 336b), but the added word 'good' is pleonastic in the following context however) -- does, Plato says in another dialog (Meno 75c-d, tr. Jowett: "And if he was a philosopher of the eristic and antagonistic sort, I would say to him: You have my answer, and if I am wrong, your business is to take up the argument and refute me. But if we were friends ["the dialectician who is also a good companion"], and we were talking as you and I are now, I would reply in a milder strain and more in the dialectician's vein; that is to say, I would not only speak the truth, but I would make use of premises which the person interrogated would be willing to admit." Cf. the description in Xenophon's Memorabilia (iv, 6, 14-15) of Socrates' always seeking step-by-step agreement in dialectic). Philosophy is not eristic (but when "unreadable sentences" (Malcolm) are allowed to pass for philosophy, that is akin to the eristic Plato describes; cf. also Sophist 243b, for what Plato describes there, namely, indifference to being understood by those outside one's own school of thought, is also akin to eristic) -- and Sophistry is not Philosophy, but in Plato's time that distinction was not clear to many or most people, as e.g. the discussion with Callicles (Gorgias 481b ff.) shows; cf. Plato's Apology, where apparently most of the jurors apparently did not know how to distinguish one from the other.

Thales and the maidservant, Plato and the body

173e-174b - ... it is really only his body that sojourns in the city, while his thought ... takes wings, as Pindar says, "beyond the sky ..." ... the story of the Thracian maidservant who exercised her wit at the expense of Thales, when he was looking up to study the stars and tumbled down a well. She scoffed at him for being so eager to know what was happening in the sky that he could not see what lay at his feet. Anyone who gives his life to philosophy is open to such mockery.... he spends all his pains on the question, what man is, and what powers and properties distinguish such a nature from any other.

In Phaedrus 230a that is what Socrates says about "Know thyself"; and he says the same in Xenophon's Memorabilia iv, 2, 24-26 and iv, 7, 2-3. -- However, what Socrates does not say is that "his thought has taken wings beyond the sky"; those are Plato's thoughts.

When Plato adds "what powers and properties distinguish such a nature from any other", he is demanding the second part of a Socratic definition (according to Plato and Aristotle). Should we say that Plato is seeking a "definition of man"?

Wittgenstein's interest is decidedly with what "lays at his feet", because he believes that this is all philosophy is able to take an interest in. I don't know if he is correct (although metaphysics does seem to reduce to idle pictures), but that vision of philosophy certainly lacks the nobility of Plato's view.

175c - [The philosopher draws his companion upward to where he consents] to drop the question, "What injustice have I done to you or you to me?" and to think about justice and injustice in themselves, what each is, and how they differ from one another and from anything else ...

176b - ... the good must always have its contrary [namely, evil; but evil belongs only to the region of our mortal nature, not to the divine world]. That is why we should make all speed to take flight from this world to the other, and that means becoming like the divine so far as we can, and that again is to become righteous with the help of wisdom.

176c - ... nothing is more like the divine than any one of us who becomes as a righteous as possible.... to know this is wisdom and excellence of the genuine sort; not to know it is to be manifestly blind and base.... It is here that a man shows his true spirit ...

"... and how little is achieved when these problems are solved." -- Well, logic appears little when compared to the question [problem] of self-control, which is the "riddle of happiness" (which is, I imagine, no more than a restatement of "the riddle of existence"). Although he cast it in religious terms, although he did see it in terms of a "weakness of the will", not as a matter of the absence of knowledge, we could say that what Wittgenstein wanted was self-control.

177e - We will suppose, then, that he does not mean the name, but has in view the thing that bears it.

And the bearer of a name, if the bearer be real, must be an object, must it not, if not a visible one then an invisible one. And this is, again and again, the picture of language that Wittgenstein's logic of language is directed against. It is the fundamental question of logic or philosophical grammar, i.e. of the distinction between sense and nonsense. For a name not to have a bearer is, according to Plato, for the name to be nonsense.

179a-b - Then we may quite reasonably put it to [Protagoras] that he must admit that one man is wiser than another, and that the wiser man is the measure, whereas an ignorant person ... is not in any way bound to be a measure ...

179c-d - But with regard to what the individual experiences at the moment -- the source of his sensations and the judgments in accordance with them ... maybe they are unassailable, and those who assert that they are transparently clear and are instances of knowledge may be right, and Theaetetus was not beside the mark when he said that perception and knowledge were the same thing.... We must, then, look more closely into the matter ...

If "individual experiences at the moment" were knowledge, then I could say that I know I have a toothache. Wittgenstein will counter that 'I know' means 'I have [sufficient or compelling] grounds', and that I have no grounds for asserting that I have a toothache. (PI § 246) (On the one hand, 'know' is a tool; what work do you want it to do. But on the other hand, 'know' is a word of language -- what actual use do we make of it: in what circumstances would it make sense to say 'I know I have a toothache'?)

181b - ... if we suppose that nobodies like us ourselves can make any contribution after rejecting these paragons of ancient wisdom.

Of course there are no "nobodies" in philosophy, because the only authority in philosophy is reason, and each of us must answer to it ourselves: we cannot stand on anyone else's forehead to jump higher than our own. Each of us must question everything for ourselves, accepting nothing that does not before us stand up to the tests of reason and of the evidence.

Can we know the name of anything if things are as Heraclitus says they are?

181d - So I should recognize these as two kinds of change -- alteration and local motion. [All motion is change, but not all change is motion; in the view of Heraclitus everything is constantly changing ("in flux") -- nothing is permanent (nothing is "at rest".]

182d - [If everything is constantly changing, how can anything be called by its right name?]

183a-b - ... if all things are in change, any answer that can be given to any question is equally right; you may say it is so and it is not so -- or "becomes", if you prefer to avoid any term that would bring these people [who say that everything is always changing] to a standstill.... Except, Theodorus, that I have used the words 'so' and 'not so', whereas we have no right to use this word 'so' -- what is "so" would cease to be in change ... Some new dialect will have to be instituted for the exponents of this theory, since, as it is, they have no phrases to fit their fundamental proposition -- unless indeed it were 'not even nohow'.

185c-e - THEAETETUS: You mean existence and nonexistence, likeness and unlikeness, sameness and difference [cf. Sophist 255], and also unity and numbers in general as applied to them, and clearly your question covers "even" and "odd" and all that kind of notions. You are asking through what part of the body our mind perceives these?

186d - ... knowledge does not reside in the impressions [sense perceptions], but in our reflection upon them. It is there, seemingly, and not in the impressions, that it is possible to grasp existence and truth.

But I do not (normally) infer that I am seeing rain: I simply see that it is raining when I look out the window. And that is one of the many things we call 'knowledge'. Of course a lot of background is needed in order for me to know that it is raining; I have e.g. to have learned a language, and to have learned to apply the word 'rain'. But, then, does a dog not know that it is raining. Look at the word 'know' as a tool that has some uses in our life: we are not investigating a phenomenon but instead a concept PI §§ 421, 360, 90a, 383). We are not seeking a "theory of knowledge" -- i.e. a statement of "what knowledge really is", but criteria for applying the word 'knowledge' (Of course that is Wittgenstein's view of what we are doing, not Plato's).

Arriving either at knowledge, or else at the condition of Socratic ignorance

187c - If we go on like this, either we shall find what we are after, or we shall be less inclined to imagine we know something of which we know nothing whatever, and that surely is a reward not to be despised. [cf. Theaetetus 210b-c.]

Cf. "Those who think they know what they don't know are themselves misled and mislead others." (Xenophon, Memorabilia iv, 6, 1)

189d - [False judgment is: mistaking one existent for another existent.] [cf. "But if we are thinking, mustn't we be thinking about something existent?" -- "Necessarily." -- Wittgenstein quotes Theaetetus 189a in Philosophical Investigations § 518.]

"... when the mind is thinking it is simply talking to itself"

189e-190a - And do you accept my description of the process of thinking? -- How do you describe it? -- As a discourse which the mind carries on with itself about any subject it is considering.... I have a notion that, when the mind is thinking it is simply talking to itself, asking questions and answering them, and saying yes or no.... So I should describe thinking as discourse, and judgment [i.e. reaching a decision] as a statement pronounced, not aloud to someone else, but silently to oneself.

In Cornford's translation, when Socrates says 'notion', this seems to contrast with 'knowledge'. Above he says, "I have a notion" [cf. "I have a surmise"], meaning that this is not something that has been tested and found to be true in a philosophical investigation (dialectic). But cf. Theaetetus 202b: "when a man gets hold of a true notion of something".

If "the mind is talking to itself" -- but the word 'mind' is not the name of an object or a location (of any kind) -- then are not words being used, and therefore does not Plato mean that "thinking is operating with signs" (cf. BB p. 6, 15-16, although note that we call various things (e.g. a carpenter sizing up a piece of wood) thinking that do not use words)?

Talking silently or even aloud to oneself is one of the things we call 'thinking', and the grammars of 'thought' and 'mind' are interconnected -- but their relation is not that of agent to product: the production of thoughts is what we call 'thinking': thoughts "occur" to one (And that, like all of these remarks is a grammatical remark, not a "theory about the agent of thought" but a reminder of how we normally use the word 'thinking' when we are not following false grammatical analogies). The relation between the grammar of the word 'mind' and the grammar of the word 'thinking' is the relation of two words belonging to the same part of speech (or, grammatical category), in this case psychological-words (as contrasted with physiological-words, for example, where the false analogy would be to construe the grammar of 'mind' to be the same as the grammar of e.g. the word 'brain' or 'hand').

The relationship of the grammars of the words 'mind' and 'thought' (or 'thinking') is similar to [-- i.e. can be compared in such-and-such a way, namely: just as sadness if not the cause of crying, mind is not the cause of thought --] the relationship between the grammars of the words 'sadness' and 'crying' (The word 'crying' is used both as a noun and as a verb). That is, "the mind" no more produces thoughts ("the mind" is not the agent of thinking) than sadness produces crying (Sadness is not the agent of crying): if someone cries because they are sad, "the 'because' does not relate to a cause" (RPP i § 217), just as, if someone thinks because they have a mind, the word 'because' does not here indicate a cause: That word functions instead as a grammatical reminder of the rules of sense and nonsense (the logic) of our language -- i.e. the proposition 'If someone does not have a mind, they cannot think' is not a statement of fact but instead a rule of grammar.

We might define the word 'mind' in some contexts as 'a collection of thoughts' (as in other contexts, 'of feelings'), as e.g. when we read Plato's Theaetetus we are reading Plato's thoughts, something which if the word 'mind' were the name of an object it would be nonsense (i.e. an undefined combination of words) to say that we could do. Wittgenstein tried to invent a meaning (cf. BB p. 6-7) for questions such as: where is "location of the mind"? The paper on which the words are written; what does the thinking? The hand that writes the words.

"... talking to itself, asking questions and answering them" -- Thinking as question and answer. That would be Socratic thinking ("holding discourse with oneself", dialectic). And I am sorry to say that my thinking very seldom takes the form of asking myself questions: I simply make assertions, "like someone who has no real education and cares nothing for the facts of the case" (Phaedo 91a). Even my skepticism is emphatic! On the other hand, remember that a thesis is often the source of a question (as in dialectic's cross-questioning), so that beginning with an assertion is not a false start -- although to go no further is a false end.

The concept 'thing', that most general of all categories, is often a sanctuary for ignorance, a place for it to hide. I don't think I used the word 'thing' that way above, however, because it does not there mask a theory.

Plato's criticism of Socrates' definition of 'knowing'

Note.--There is a brief summary of Plato's criticism in this dialog of the historical Socrates, written many years after the other comments on this page were.

195a - "narrow mind" [Here in the sense of someone with not much space to put things]

196d-e - ... our whole conversation from the outset has been an inquiry after the nature of knowledge on the supposition that we did not know what it was.... Then, doesn't it strike you as shameless to explain what knowing is like, when we don't know what knowledge is?... Times out of number we have said, 'we know', 'we do not know', 'we have knowledge', 'we have no knowledge', as if we could understand each other while we still know nothing about knowledge. At this very moment, if you please, we have once more used the words 'know nothing' and 'understand', as if we had a right to use them while we are still destitute of knowledge.

198a-b - [Arithmetic], I take it, is the science in virtue of which a man has in his control pieces of knowledge about numbers and can hand them over to someone else [Plato is considering here the notion that "knowing is possessing knowledge" (197a-b)].... And when he hands them over, we call it 'teaching', and when the other takes them from him, that is 'learning', and when he has them in the sense of possessing them in that aviary [This being a picture of the mind (197c)] of his, that is 'knowing'.

True belief is knowledge

200d-e - Then tell me, what definition can we give [of 'knowledge'] with the least risk of contradicting ourselves?

THEAETETUS: That true belief is knowledge.

201b-d - SOCRATES: And when a jury is rightly convinced of facts which can only be known by an eyewitness, then, judging by hearsay and accepting a true belief, they are judging without knowledge ... But if true belief and knowledge were the same thing, the best of jurymen could never have a correct belief without knowledge. It now appears they must be different things.

THEAETETUS: Yes, Socrates, I have heard someone make the distinction.... He said that true belief with the addition of an account (logos) was knowledge, while belief without an account was outside its range. Where no account could be given of a thing, it was not 'knowable' -- that was the word he used -- where it could, it was knowable.

SOCRATES: A good suggestion. But tell me how he distinguished these knowable things from the unknowable.

Knowledge as "justified true belief"

But 'knowledge' and 'belief' are different concepts: knowledge is not a type (or, sub-class) of belief (neither of true nor of false belief), and neither 'knowledge' nor 'belief' is a psychological concept, and neither is a Socratic "account" a state of mind.

The "someone" Theaetetus heard appears to resemble the historical Socrates himself, but it is not that Socrates, for the historical Socrates uses 'account' ('explanation') very differently from the way Plato does in this dialog (see Xenophon, Memorabilia iv, 6, 1, where 'account' simply means a proposition that can be tested in discussion). The irony is that in Socrates' sense of 'an account', rendering an account of what one thinks one knows is exactly what in this dialog Theaetetus tries to do, to explain what he knows, or thinks he knows, to another, namely to his companion Plato's literary Socrates. In Plato's own words, "And that which we know we must surely be able to tell?" (Laches 190c, tr. Jowett) To "tell" is to "give an account".

Being able to state what one knows and defend one's statement against refutation in Socratic dialectic (cross-questioning in dialog) is the standard Socrates sets for knowledge in philosophy: This ability, if demonstrated, is what Socrates calls 'philosophical knowledge'. [Must we be able to say what we know?] Socrates' standard says "what counts for, what against" anyone's claim to know a thing. (For Plato, however, that standard is insufficient: for Plato, 'to know a thing' is 'to be able to state what the thing's essence is', e.g. the particular common nature named by a particular common name.)

I have made preliminary criticism of the academic notion that "knowledge is justified true belief", on the grounds that the addition of the word 'belief' to the formula neither makes anything clearer nor indeed has any meaning -- because 'knowledge' and 'belief' are distinct concepts, different concepts -- i.e. we use those two words differently: knowledge is not a subclass of belief (It is not a type or a degree of belief). (By 'concept' I mean 'a grammar', or, 'set of rules for using a word'.)

Furthermore, and in any case, neither knowledge nor belief is a state of mind, which is what the academic imagines is the meaning of the words 'knowledge' and 'belief' in the formula "knowledge is justified true belief". (Neither are those words the names of dispositions, though it's true that one does not cease knowing or believing when one is asleep.)

For a statement (or, proposition) to be counted as knowledge it must be justified "and it must be true" -- but how can it be true other than by being justified? There is no other criterion for a statement's being true. And therefore "it must be true" is tautological [pleonasm] to "it must be justified", because if it is justified, it is true. Which amounts to saying that to count as knowledge, a statement must be an account that stands up against the tests of experience and reason when cross-questioned. That is how the truth of a statement is justified. And further whether or not a proposition is true has nothing to do with whether any individual believes it or doesn't believe it to be true.

In sum, Plato's "true belief" clause adds nothing to Socrates' definition of 'knowledge' = 'being able to give an account of what one knows to others', or in other words, 'being able to justify [defend] one's statement when it is tested in cross-questioning'.

[Plato's notion that "knowledge is really the soul's direct contact with the Forms" is a picture belonging to metaphysics, not logic of language. It is not a picture that shows us our normal concept 'knowledge'.]

202b-c - So when a man gets hold of the true notion of something without an account, his mind does think truly of it, but he does not know it, for if one cannot give and receive an account of a thing, one has no knowledge of that thing. But when he has also got hold of an account ... he is fully equipped with knowledge.

Plato here introduces something similar to the notion of "indefinable signs" (202a-b), the notion that a definition (explanation of meaning) must be a verbal formula, a notion which has crippled understanding ever since. However, rather than Plato's notion, what is important here is the question Socrates asks in Laches 190c: "And that which we know we must surely be able to tell?" (tr. Jowett) -- i.e. must we be able to put what we know into words? Wittgenstein gives this example: "knowing how a clarinet sounds" (PI § 78), which seems to show that there is no essence of knowledge -- i.e. that we call a variety of [different] things 'knowledge', although I nonetheless argue that the grammars of the words 'knowledge' and 'justification' are essentially connected.

201e-202b - ... the first elements of which we and all other things consist are such that no account can be given of them. Each of them just by itself can only be named.... But in fact there is no formula in which any element can be expressed; it can only be named, for a name is all there is that belongs to it.... Accordingly, elements are inexplicable and unknowable, but they can be perceived ...

But when we come to things composed of these elements, then, just as these things are complex, so the names are combined to make a description (logos), a description being precisely a combination of names.

Wittgenstein quotes Theaetetus 201e-202b in the context of his criticism of his own Tractatus Logico-Philosophicus's notion of "absolutely simple objects" that can only be named. (PI § 46)

"... a description being precisely a combination of names." -- That is what proposition is said to be in the earlier book. But obviously 'the' is not a name, and therefore the Tractatus's "propositions", being nothing but a string of names -- i.e. of names of "the first elements", do not much resemble normal propositions.

202d-e - ... for how can there ever be knowledge without an account and right belief? But there is one point in the theory as stated that does not find favor with me.... [Namely,] It says that the elements are unknowable, but whatever is complex ("syllables" [202b has "complexes ("syllables")"]) can be known.... [Plato offers an illustration (202e), namely,] Letters -- the elements of writing -- and syllables [By 'syllable' is meant: a pronounceable combinations of letters, such as e.g. the 'so' of 'Socrates' (203a)]. That and nothing else was the prototype the author of this theory had in mind, don't you think?

Plato, or his translator [I myself do not know Greek], speaks of a "theory of knowledge". What might that mean -- is a grammatical investigation theory-making? If we state a definition, if our definition is to be a report of how do we use the word 'knowledge', we want our definition to "account for all the data", and is that not what a theory tries to do? But here we run into the equivocal use of the word 'theory'; in the case of grammar, 'theory' = 'hypothesis', where 'hypothesis' means 'verifiable statement of fact' (as opposed to Drury's "models, pictures, maps"). Were grammar theoretical rather than determinate, there would be no clear and sharp distinction between sense and nonsense (One might speak of probability instead: "This is probably meaningless, but one never knows for sure").

Plato, of course, will not allow that he is conducting a mere grammatical investigation (168b-c), which Russell characterized as an investigation of "the different ways in which silly people can say silly things" (My Philosophical Development, New York: 1959, p. 230); instead, Plato wants to seek the essence of knowledge. But is there an essence of knowledge; -- and what other than a grammatical investigation can determine that? In fact, we use the word 'know' in a variety of different ways, and it is Plato's refusal to accept those differences which makes it impossible for him to "make out to my own satisfaction what knowledge is" (146a).

203a-c - Did we learn our letters on that principle or not? To begin with, is it true that an account can be given of syllables but not of letters? [Asked to give "an account" of the first syllable of 'Socrates', Theaetetus answers "S and O".] Go on, then, give me a similar account of [the letter] S.

THEAETETUS: But how can one state the elements of an element?

SOCRATES: But now, have we been right in declaring that the letter cannot be known, though the syllable can?

In this case 'to give an account' means to give an analysis -- i.e. to break a complex (or, compound) down into its parts. This seems to take us very far from Socrates' notion of giving an account -- i.e. of stating a definition. Or does it? Suppose we ask "What is the meant by 'the syllable SO'?" Would it not be a definition -- i.e. an explanation of meaning -- to answer 'a combination of the letters S and O' [or as we would say 'of the sounds S and O]. Then one meaning of the word 'definition' is -- i.e. one way that we use that word is to mean: 'the break down of a complex into it the parts' (as one meaning of the word 'know' is: "being able to reply to the question, what any given thing is, by enumerating its elements" (206e-207a)).

There are many meanings of the word 'meaning' -- as well as of 'to give an account' -- not just one. And that is the point not made by Plato.

205c-d - [What have been called 'elements' or 'primary things' are "simple in nature and indivisible into parts" and therefore "inexplicable and unknowable" [if the only way to know a thing is to analyze it]. However, that does not appear to be a true account.]

205e-206b - ... would not your own experience in learning your letters rather incline you to accept the opposite view?... that all the time you were learning you were doing nothing else but trying to distinguish by sight or hearing each letter by itself, so as not to be confused by any arrangement of them in spoken or written words. [And similarly, at music school you learned the individual notes that are "the elements of music".] Then, if we are to argue from our own experience of elements and complexes to other cases, we shall conclude that the elements in general yield knowledge that is much clearer than knowledge of the complex and more effective for a complete grasp of anything we seek to know. [Thus if anyone tells us that the elements are unknowable, we shall suppose that] he is playing with us.

206c - Well, then, what is the term 'account' intended to convey to us?

208c - SOCRATES: The meaning most people would give [to 'account'] -- being able to name some mark by which the thing one is asked about differs from everything else.... 209a - Suppose I have a correct notion about you [Theaetetus]; if I add to that the account of you, then, we are to understand, I know you. Otherwise I have only a notion.... And 'account' means putting your differentness into words.

That is the second part of Socratic definition (according to Aristotle). However, by adding that you do not know something until you have put "what you have a notion of" -- i.e. what you know -- into words, Plato has chosen one sense of 'know' rather than another. He uses the word 'know' equivocally -- i.e. switches back and forth between its various senses, between the various ways we use that word -- and is perplexed because he does not find an essence of knowledge.

208c-d - THEAETETUS: Could you give me an example of such an account of a thing?

SOCRATES: Take the sun as an example. I dare say you will be satisfied with the account of it as the brightest of the heavenly bodies that go round the earth.... Let me explain the point of this example. It is to illustrate what we were just saying -- that if you get hold of the difference distinguishing any given thing from all others, then, so some people say, you will have an "account" of it, whereas, so long as you fix upon something common to other things, you account will embrace all things that share it.

208e - And if, besides a right notion of a thing, whatever it may be, you also grasp its difference from all other things, you will have arrived at knowledge of what, till then, you had only a notion of.

But what does Plato mean here by "a right notion of a thing"? What he means is: knowledge of a thing, because he goes on to say that "the correct notion of anything must itself include the differentness of that thing" (209d) because how can Socrates have a "correct notion" of Theaetetus if he does not know how Theaetetus is different from Theodorus or any other man in the street (209b).

208e - Really, Theaetetus, now I come to look at this statement at close quarters, it is like a scene painting. I cannot make it out at all, though, so long as I kept at a distance, there seemed to be some sense in it.

209e-210a - If the direction to add an "account" means that we are to get to know the differentness, as opposed to merely having a notion of it, [then this "definition of knowledge" is silly] because 'getting to know' means acquiring knowledge ... So, apparently, to the question, "What is knowledge?" our definition will reply, "Correct belief together with knowledge of a differentness" ... and when we are inquiring after the nature of knowledge, nothing could be sillier [than a definition like that (i.e. a "definition" of the form: "knowledge is knowledge")].

210a-b - So, Theaetetus, neither perception, nor true belief, nor the addition of an "account" to true belief, can be knowledge.

The effect of not thinking you know what you don't know

210b-c - [But as "a consequence of today's scrutiny", Theaetetus] you will be gentler and more agreeable to your companions, having the good sense not to fancy you know what you do not know [cf. Sophist 230c-d]. For that, and no more, is all that my art [(of midwifery (150b-c)] can effect ...

For that is all that my art can accomplish

But if, Theaetetus, you should ever conceive afresh [Again, the image is of an egg, the present one having proved to be mere wind rather than a chick (151e)] you will be all the better for the present investigation, and if [you do] not [conceive another egg, i.e. thesis], you will be soberer and humbler and gentler to other men, and will be too modest to fancy that you know what you do not know. These are the limits of my art; I can no further go ... (210c-d, tr. Jowett)

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