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The Theaetetus (Greek: Θεαίτητος) is one of Plato's dialogues concerning the nature of knowledge. The framing of the dialogue begins when Euclides tells his friend Terpsion that he had written a book many years ago based on what Socrates had told him of a conversation he had with Theaetetus when Theaetetus was quite a young man. (Euclides also notes that he had had to go back to Socrates to ask some more questions about the speeches due to his spotty recollection of the account.)
Euclides is prompted to share his book when Terpsion wonders where he had been: Euclides, who apparently can usually be found in the marketplace of Megara, was walking outside of the city and had happened upon Theaetetus being carried from Corinth to Athens with a case of dysentery and a minor war wound; Euclides remarks that Socrates had made some uncanny predictions about Theaetetus needing to rise to fame. Euclides' book is read aloud to the two men by a slave boy in the employ of Euclides.
In this dialogue, Socrates and Theaetetus discuss three definitions of knowledge: knowledge as nothing but perception, knowledge as true judgment, and, finally, knowledge as a true judgment with an account. Each of these definitions is shown to be unsatisfactory. The conversation ends with Socrates' announcement that he has to go to court to answer to the charges that he has been corrupting the young and failing to worship Athenian gods.
Midwife to knowledge 
Socrates asks Theodorus if he knows of any geometry students who show particular promise. Theodorus assures him that he does, but that he does not want to over-praise the boy, lest anyone suspect he is in love with him. He says that the boy, Theaetetus, is a young Socrates look-alike, rather homely, with a snub-nose and protruding eyes. The two older men spot Theaetetus rubbing himself down with oil, and Theodorus reviews the facts about him, that he is intelligent, virile, and an orphan whose inheritance has been squandered by trustees.
Socrates tells Theaetetus that he cannot make out what knowledge is, and is looking for a simple formula for it. Theaetetus says he really has no idea how to answer the question, and Socrates tells him that he is there to help. Socrates says he has modelled his career after his midwife mother. She delivered babies and for his part, Socrates can tell when a young man is in the throes of trying to give birth to a thought.
Philosophical labor 
Socrates thinks that this idea must be identical in meaning, if not in actual words, to Protagoras' famous maxim "Man is the measure of all things." Socrates wrestles to conflate the two ideas, and stirs in for good measure a claim about Homer being the captain of a team of Heraclitan flux theorists. Socrates dictates a complete textbook of logical fallacies to the bewildered Theaetetus. When Socrates tells the child that he (Socrates) will later be smaller without losing an inch because Theaetetus will have grown relative to him, the child complains of dizziness (155c). In an often quoted line, Socrates says with delight that "wonder (thaumazein) belongs to the philosopher". He admonishes the boy to be patient and bear with his questions, so that his hidden beliefs may be yanked out into the bright light of day.
Examining the offspring 
When Socrates sums up what they have agreed on so far, it becomes problematic that knowledge is sense perception, for Socrates raises the question that "When the same wind blows, one of us feels cold and the other not?" As a result he introduces the idea of Heraclitean flux to act as a defense to the wind objection. Heracliteanism shows that "Nothing is in itself just one thing...Everything is in a process of coming to be". Thus as there is no fixed meaning in things, but they draw their meaning in a referential difference to other things, the wind objection can be incorporated into Theaetetus's claim that "Knowledge is sense perception". As a result they can then continue their inquiry as to the truth of this claim. It is important to note that the Heraclitean doctrine of Flux is not the same as the Protagorean doctrine. The Protagorean is radical truth relativism whereas the Heraclitean is radical reality relativism. It serves as a supporting theory to the Protagorean interpretation of Theaetetus's claim, in order that they might fully inquire as to the validity of this premise. Socrates admits that it is unfortunate that Protagoras is dead and cannot defend his idea against people such as himself. He says that the two of them are "trampling on his orphan" (164e) but the charge remains.
Abusing the "orphan" of Protagoras 
Since Protagoras is dead, Socrates puts himself in the sophist's shoes and tries to do him the favor of defending his idea (166a-168c). Socrates continues to find more ways to misinterpret and misrepresent him - "mistreat his orphan." Putting words in the dead sophist's mouth, Socrates declares that Protagoras asserts with his maxim that all things are in motion and whatever seems to be the case, is the case for the perceiver, whether the individual or the state.
At the end of his speech, Socrates admits to Theodorus that if Protagoras were alive to defend his idea, he would have done a far better job than Socrates has just done. Theodorus tells Socrates that he must be kidding, that he has come to the task with boyish vigor. Theodorus does not claim to be a disciple of Protagoras, but never contradicts Socrates repeated assertions that he is a friend of Protagoras. Socrates admits he has used the child's timidity to aid him in his argument against the doctrine of Protagoras (168d).
Socrates, not at all certain that he has not misrepresented Protagoras in making each man the measure of his own wisdom, presses Theodorus on the question of whether any follower of Protagoras (himself included) would contend that nobody thinks anyone else is wrong (170c). Theodorus proves to be helpless against Socrates' confusions. He agrees that Protagoras concedes that those who disagree with him are correct (171a). In making Protagoras a complete epistemological relativist, where every person's individual perceptions are his reality and his truth, both Socrates and Theodorus paint Protagoras as maintaining an absurd position. Socrates says that if Protagoras could pop his head up through the ground as far as his neck, he would expose Socrates as a speaker of nonsense, sink out of sight, and take to his heels (171d).
The absent-minded philosopher 
Socrates then proceeds to explain why philosophers seem clumsy and stupid to the common lot of humanity. Socrates explains that philosophers are open to mockery because they are not concerned about what interests most people: they could not care less about the scandals in their neighbor's house, the tracing of one's ancestry to Heracles, and so on. In contrast, the philosophy is concerned with things that are, such as beauty and knowledge, which are "truly higher up". It is here that Socrates draws the classic portrait of the absent-minded intellectual who cannot make his bed or cook a meal (175e). Socrates adds a big bifurcation to this speech, saying that there are only two kinds of lives to be lived: a divinely happy one, lived by righteous philosophers or a godless, miserable one, such as most people live (176-177). Socrates admits this was a digression that threatens to drown his original project, which was to define knowledge. Theodorus, the old geometer, tells Socrates that he finds this sort of thing easier to follow than his earlier arguments.
The men of flux 
Socrates says that the men of flux, like Homer and Heraclitus, are really hard to talk to because you can't pin them down. When you ask them a question, he says, they pluck from their quiver a little aphorism to let fly at you, and as you try to figure that one out, they wing another one at you. They leave nothing settled either in discourse, or in their own minds. Socrates adds that the opposite school of thought, that teaches of the "immovable whole" is just as hard to talk to (181a,b). Socrates says he met the father of the idea, Parmenides, when he was quite young, but does not want to get into another digression over it.
The mind as a bird cage 
Perhaps the most delightful talk in the dialogue comes near the end, when Socrates compares the human mind to a birdcage. Socrates draws the distinction between having and possessing; the former typically implies the latter, thought on the other hand, one can possess something, such as a bird, without actually having it (with them at any moment)(199a). Socrates says that as a man goes hunting about in his mind for knowledge of something, he might grab hold of the wrong thing. He says that mistaking eleven for twelve is like going in for a pigeon and coming up with a dove (199b). Theaetetus joins in the game, and says that to complete the picture, you need to envision pieces of ignorance flying around in there with the birds. But if this is the case, how would you be able to distinguish between the birds representing real knowledge and the ones representing false ones? Are there other birds that represent this type of knowledge? Socrates comes to the conclusion that this is absurd and therefore he discards the birdcage analogy.
Socrates and the Jury 
After discarding the bird-cage analogy, Socrates and Theaetetus return to the definition of knowledge as 'true judgement' (200e). This, Theaetetus argues, is true because it is 'free from mistakes' (200e). However Socrates introduces an example of a jury in the law-courts, being persuaded of an opinion by a lawyer. This persuasion is not the same as knowing the truth, as all is produced is 'conviction' in judging whatever the lawyers want (201a). Although Theaetetus hopes it is possible the lawyer will be able to 'persuade' the jury of the truth (201b), Socrates is unsatisfied as if they are justly persuaded, they will have true knowledge. However, in Socrates' belief, they cannot make a correct judgement as they would not have true knowledge (201c). With this conflict, Socrates decides that true judgement and knowledge must be different things.
Knowledge as judgement with an account 
After distinguishing between knowledge and true judgement, Theaetetus recalls being told that true judgement 'with an account (logos) equates to knowledge (201d). Things without an account are 'unknowable', while things with an account are 'knowable'.
Socrates responds by telling of a dream, in which he overheard people talking of primary elements (201e). These primary elements can only be named, they cannot be thought of as existing or not - he gives examples of words like 'itself, or that, each, alone or this' (202a). While they can be added to other words, they by themselves are just a name. When these elements are added together, Socrates says that a 'complex' is formed (202b). The primary elements are 'unaccountable and unknowable, but perceivable' while the complexes are 'knowable and expressible' and so can be objects of 'true judgement' (202b). He concludes his dream by agreeing with Theaetetus that knowledge is 'true judgement with an account' (202c).
However, Socrates exposes some difficulties by examining letters. He takes the first two letters of his name, S and O to wonder if the syllable 'So' is knowable while the individual letters are not (203b-d). Theaetetus finds the idea strange, so Socrates deduces that in order to know the syllable, the letters must be known first (203e). Socrates proposes that the syllable can be a 'single form' produced from the letters. With this in mind, Socrates considers whether the 'sum' and the 'whole' are the same (204a). Theaetetus initially says they are not, but changes his mind in confusion when Socrates leads him through maths and the different ways of expressing the number six (204c-205b). After agreeing this, Socrates returns to the subject of syllables and letters to conclude from Theaetetus' answers that syllables are different from letters and cannot contain letters (205b). Theaetetus admits this idea is ridiculous (205c). Socrates returns to talking about elements and complexes to propose that they are in the same class, as they have 'no parts and [are] a single form' (205d).
Socrates sums up this reversal by remarking that if anyone tries to tell them the complex is knowable and expressable while the element is the opposite, 'we had better not listen to him' (205e). He cites the example of a musician distinguishing individual notes (conceded to be elements of music) to propose that elements are 'much more clearly known'(206b).
Socrates proposes an account to be 'making one's thought apparent vocally by means of words and verbal expressions' (206d). However, he wonders if that is so, everyone will be able to make judgement 'with an account' as they can all (except for the deaf and dumb) vocalize and express opinions on matters (206e). Socrates examines it further by suggesting that a man who can vocalize his judgement must be able to make reference to the primary elements of the subject (207a). Giving an example of defining a wagon by its individual parts (207a), agreement is reached that an account is 'going through a thing element by element'(207d). Socrates questions Theaetetus by drawing on his learning of how to write, and the idea that if you misplace individual elements (letters) of a name, that does not mean you have knowledge of it (208a). This finishes Socrates' second definition of an account as 'the way to the whole through the elements' (208c). The third definition Socrates offers is 'being able to tell some mark by which the object you are asked about differs from all other things' (208c), giving the example that the Sun is distinct for its brightness. However, this definition of an account fails as by getting to know the differentness of an object, you have to acquire knowledge about it. Thus the answer to the initial question 'What is knowledge' would be heavily circuitous - correct judgement accompanied by 'knowledge' of the differentness, which Socrates admits is 'silly' (210a).
Socrates concludes the dialogue by announcing that all the two have produced is mere "wind-eggs" and that he must be getting on now to the courthouse to face his trial being brought against him by Meletus.
Significant references in the dialogue 
In this dialogue, Socrates refers to Epicharmus of Kos as "the prince of Comedy" and Homer as "the prince of Tragedy", and both as "great masters of either kind of poetry".[note 1] This is significant because it is one of the very few extant references in greater antiquity (Fourth century BC) to Epicharmus and his work. Another reference is in Plato's Gorgias dialogue.
- "Summon the great masters of either kind of poetry- Epicharmus, the prince of Comedy, and Homer of Tragedy", Theaetetus, by Plato, section §152e. (translation by Benjamin Jowett). There is some variability in translation of the passage. Words like "king", "chief", "leader", "master" are used in the place of "prince" in different translations. The basic Greek word in Plato is "akroi" from "akros" meaning topmost or high up. In this context it means "of a degree highest of its kind" or "consummate" (cf. Liddell & Scott, A Greek-English Lexicon).
Selected secondary literature 
- Cornford, F.M., "Plato's Theory of Knowledge: The Theaetetus and The Sophist". Dover, 2003 [first published in 1935].
- Desjardins, Rosemary, "The Rational Enterprise: Logos in Plato's Theaetetus", SUNY, 1990.
- Doull, James (1977). "A Commentary on Plato's Theaetetus". Dionysius I: 5–47.
- Klein, Jacob, "Plato's Trilogy: Theaetetus, The Sophist and the Statesman". University of Chicago Press, 1977.
- Benardete, S., Commentary to Plato's Theaetetus. Chicago: University of Chicago Press, 1984.
- Burnyeat, M.F., The Theaetetus of Plato (with a translation by Jane Levett). Hackett, 1990.
- Campbell, L., The Theaetetus of Plato. Oxford University Press, 1883.
- Heidegger, M., The Essence of Truth. Continuum, 2002.
- The full text is available from Project Gutenberg or the Perseus Project in both Greek and English.
- Plato on Knowledge in the Theaetetus entry by Timothy Chappell in the Stanford Encyclopedia of Philosophy
- Cornford, F.M., "Plato's Theory of Knowledge" at googlebooks
- A Discussion of Theaetetus' Contributions to Euclid's Elements