Theaetetus (dialogue)

Socrates thinks that the idea that knowledge is perception 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 [lower-alpha 1]. 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. 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" [lower-alpha 2] but the charge remains. Since Protagoras is dead, Socrates puts himself in the sophist's shoes and tries to do him the favor of defending his idea.[lower-alpha 3] Socrates concedes that if Protagoras were still alive, he would have more to say in his own defense, and that they are now essentially mistreating "his orphan child." 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 Protagoras would have done a far better job of defending his own ideas. 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 states that he was a friend. Socrates invites Theodorus to put up a more vigorous defense of Protagoras, as he does not want it suggested that he has used the child's timidity (of Theaetetus) to aid him in his argument against the doctrine of Protagoras.[lower-alpha 4]. 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.[lower-alpha 5] Theodorus proves to be helpless against Socrates' arguments. He agrees that Protagoras concedes that those who disagree with him are correct.[lower-alpha 6] 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 recounts the tale of the philosopher who was so caught up in looking at the stars that he fell into a well.

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 philosopher 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.[lower-alpha 7] 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. 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.[lower-alpha 8] 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.[lower-alpha 9]

Socrates compares the human mind to an aviary. Socrates draws the distinction between having and possessing; the former typically implies the latter, though on the other hand, one can possess something, such as a bird, without actually having it (with them at any moment).[lower-alpha 10] 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,[lower-alpha 11] 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 aviary analogy. After discarding the bird-cage analogy, Socrates and Theaetetus return to the definition of knowledge as 'true judgement' [lower-alpha 12]. This, Theaetetus argues, is true because it is 'free from mistakes.'[lower-alpha 13] 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.[lower-alpha 14] Although Theaetetus hopes it is possible the lawyer will be able to 'persuade' the jury of the truth,[lower-alpha 15] 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.[lower-alpha 16] With this conflict, Socrates decides that true judgement and knowledge must be different things. After distinguishing between knowledge and true judgement, Theaetetus recalls being told that true judgement 'with an account (logos) equates to knowledge.[lower-alpha 17] 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.[lower-alpha 18] 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.'[lower-alpha 19] 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.[lower-alpha 20] The primary elements are 'unaccountable and unknowable, but perceivable' while the complexes are 'knowable and expressible' and so can be objects of 'true judgement'.[lower-alpha 21] He concludes his dream by agreeing with Theaetetus that knowledge is 'true judgement with an account.'[lower-alpha 22] 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.[lower-alpha 23] Theaetetus finds the idea strange, so Socrates deduces that in order to know the syllable, the letters must be known first.[lower-alpha 24] 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.[lower-alpha 25] 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.[lower-alpha 26] 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.[lower-alpha 27] Theaetetus admits this idea is ridiculous.[lower-alpha 28] 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.'[lower-alpha 29] 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'.[lower-alpha 30] 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.'[lower-alpha 31] Socrates proposes an account to be 'making one's thought apparent vocally by means of words and verbal expressions.'[lower-alpha 32] 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.[lower-alpha 33] 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.[lower-alpha 34] Giving an example of defining a wagon by its individual parts,[lower-alpha 35], agreement is reached that an account is 'going through a thing element by element.'[lower-alpha 36] 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.[lower-alpha 37] This finishes Socrates' second definition of an account as 'the way to the whole through the elements' [lower-alpha 38].

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' [lower-alpha 39], 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' [lower-alpha 40].

Socrates concludes the dialogue by announcing that all the two have produced are mere "wind-eggs" and that he must be getting on now to the courthouse to face his trial being brought against him by Meletus.

The Theaetetus was a popular dialogue in classical antiquity. Writing in the 1st century BCE, the Middle Platonist Eudorus of Alexandria references the definition of a philosopher as a "likeness to a god"[lower-alpha 41] and uses it to justify his synthesis of Platonism with Pythagorean teachings on virtue.[7] One of Eudorus's contemporaries, Philo of Alexandria also quoted from it frequently, implying that he considered it an important work.[7] The Theaetetus was also the subject of many philosophical commentaries. The earliest surviving Platonic commentary, dating from between the 1st century BCE and the 2nd century CE, is a commentary on the Theaetetus, which goes into detail on the nature of knowledge outlined in the work and compares it to the definitions of knowledge outlined in the Meno and the Sophist.[8] The Theaetetus is also referenced frequently by Alcinous in his Handbook of Platonism.[9]

In Late antiquity, the work was included in the Neoplatonic curriculum of twelve dialogues developed by Iamblichus, where it was preceded by the Cratylus, and considered a theoretical work on logic.[10]

In the 20th century, the dialogue and its definition of knowledge as a "Justified True Belief" were investigated by Edmund Gettier, who investigated Platonic epistemology as related in the Theaetetus and the Meno is his work "Is Justified True Belief Knowledge (1963)" where he develops the Gettier problem, an example of the type of scenario which challenges the notion that a justified true belief is the same thing as knowledge.

As is standard for the works of Plato, reference numbers are to the page numbers of the 1572 edition of Henri Estienne (Stephanus). Quotations are to the edition by Benjamin Jowett, which is freely available and in the public domain.