In Plato's Theaetetus, Euclides and Terpison listen to a version of a dialogue between Socrates and the young Theaetetus that took place many years ago and is read by a slave. At the very moment the dialogue is being read, the now grown Theaetetus lies dying in Athens from a case of dysentery contracted while on campaign in Corinth. The copy of the dialogue read by the slave is not a transcript, it is rather a reconstruction, a kind of historical fiction. Euclides tells Terpison that he took some notes at the time he witnessed the discussion, but readily admits he was unable to recall the whole thing from memory. Rather he went home and wrote down his initial recollections. Later, when on occasion he would journey from his home in Megara to Athens, he would consult with Socrates and then make corrections when he went back home. In this way, over an unspecified but clearly not short period of time, the dialogue came to have the form that the two men are portrayed as hearing and that we supposedly read today, even as we know that this too is a fiction, since our author is Plato, not his imagined Euclides, let alone Socrates himself. Determinate authority is hard to locate in the Theaetetus, to say the least.

The subject of the dialogue is the nature of knowledge. Socrates approaches the young Theaetetus in the company of the geometer Theodorus and tries unsuccessfully to get each of them to offer a defensible definition of knowledge.