As Strauss notes, there are always three different layers to a Platonic dialogue. On the surface, the Theaetetus is a dialogue addressing the question of knowledge. Contextually within the larger series of dialogues, the Theaetetus is the first in a series leading up to the death of Socrates (Theaetetus, Euthyphro, Sophist, Statesman, Apology, Crito, Phaedo).
It is, first and foremost, a narrated dialogue, written by Euclides, and confirmed by Socrates. The question of the dialogue arises between a conversation with Euclides and Terpsion as they walk in search of rest on the road from Megara. Terpsion inquires about the character of Theaetetus to Euclides, as he was renowned in battle. Philosophically, the Megarians only accepted logos -that is, the account of something as truth. The dialogue suggests a problem with this school of thought, but at any rate, Euclides commands his slave boy to read the play.
Socrates was spending his time in a corner of the marketplace, shortly before his indictment. He was conversing with Theodorus, a local geometer. Per usual, Socrates is concerned with the state of philosophy, particularly among the young. Theodorus offers to Socrates a well-born young man named Theaetetus who approached with a group of other young men.
Socrates first tests Theaetetus because of the high praise received from Theodorus, as a disguised test of Theodorus in his ability to judge the youth. Socrates asks Theaetetus if we would merely accept a lyre player if he claimed to play the lyre well or if we would investigate further, to which Theaetetus responds affirmatively. This leads Socrates to a spirited discussion with Theaetetus about the nature of knowledge -“what is knowledge?” He asks at 145E about the distinction between sophia and episteme, wisdom and knowledge.
In the following eighty pages, Theaetetus gives four different accounts of the nature of knowledge -all found to be insufficient.
Definition #1: Whatever one might learn from Thoedorus in the sciences, such as geometry or shoemaking or the arts. This is found to be insufficient because, as described elsewhere by Socrates, it is a swarm of particular examples, but Socrates is looking for the whole eidos of knowledge.
Definition #2: Knowledge is nothing else than “perception” since everyone who knows something must first perceive. Socrates then discusses fixed things like hot and cold, sickness and health, dreaming versus awake, and others in the context of Heraclitean, Homeric, and Protagorean relativism. If knowledge is nothing else than perception, and everyone perceives things uniquely and distinctly, with all viewpoints being valid, than knowledge is meaningless. The image of the wax mold is invoked to show that this conception of knowledge presumes that nature stamps itself upon the human mind.
Definition #3: Knowledge is True Opinion. However, Socrates points out that opinion cannot bridge the gulf to knowledge, even if it is accidentally correct. Like someone throwing a dart at a dart board, he will accidentally be correct some of the time. This, however, does not imply that he is knowledgeable. Socrates invokes the image of hunting birds in a bird cage to show that their must be some sort of activity in the pursuit of knowledge, unlike the wax mold.
Definition #4: Knowledge is True Opinion with Speech. This is an addendum to the former question, and is spoken by Socrates, not Theaetetus in the dialogue. As the continual problem of articulating what one does or does not know is not erased by adding speech.
The dialogue ends inconclusively, as a survey of Socratic epistemological discussions. Theaetetus is made the better per Socrates’s claim that when someone is brought to the point that they realize there are things they do not know they are made more moderate. Socrates hopes to continue but he has to depart to go to his trial in response to the accusations of Meletus.