The Theaetetus is one of Plato's dialogues concerning the nature of knowledge. The framing of the dialogue begins when Euclides tells his friend Terpsion that he had written a book many years ago based on what Socrates had told him of a conversation he'd had with Theaetetus when [Theaetetus] was quite a young man. (Euclides also notes that he'd had to go back to Socrates to ask some more questions about the speeches due to his spotty recollection of the account.)