The Theaetetus (/ˌθiːɪˈtiːtəs/; Greek: Θεαίτητος) is one of Plato's dialogues concerning the nature of knowledge, written circa 369 BC.
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.
Socrates declares Theaetetus will have benefited from discovering what he does not know, and that he may be better able to approach the topic in the future. The conversation ends with Socrates' announcement that he has to go to court to face a criminal indictment.
The dialogue is framed by a brief scene in which Euclid of Megara tells his friend Terpsion that he has a written record of a dialogue between Socrates and Theaetetus, which occurred when Theaetetus was quite a young man. This dialogue is then read aloud to the two men by a slave boy owned by Euclid.
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.