In this dialogue, Socrates and Theaetetus discuss four definitions of knowledge: knowledge being the enumeration of the species 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 are 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.
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. Socrates says his work is especially difficult because he himself is barren, and, as it turns out, all the bastard notions he has helped deliver had to be killed (152b,c). Theaetetus ventures that "knowledge is nothing but sense perception".
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.
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 defence 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.
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).
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 neighbors house, the tracing of one's ancestry to Heracles, and so on. It is here that Socrates draws the classic portrait of the absent-minded intellectual who cannot make his bed, cook a meal, or drape his cloak like a gentleman (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.
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.
Perhaps the most delightful talk in the dialogue comes near the end, when Socrates compares the human mind to a birdcage. He says it is one thing to possess knowledge and another to have it about one, on hand, as it were (199a). Socrates says that as a man goes hunting about in his brain 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 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.
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". 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.