The TheŠtetus is a dialogue by Plato. In this dialogue Socrates, Theodorus of Cyrene and Theaetetus try to define what knowledge is. This conversation occurs just prior to the Trial of Socrates in 399 BC. The action of the dialogue occurs in 369 BC, when the Megarian philosophers Euclides and Terpsion have the account of the conversation read upon hearing that Theaetetus was mortally wounded in battle.
The Theaetetus belongs to a trilogy of dialogues, the others being the Sophist and the Statesman. The conversation among Socrates, Theaetetus, and Theodorus occurs immediately before the action of the Euthyphro.
Although the dialogue never succeeds in giving a clearcut answer to the question "What is knowledge?", it shows the reader some failed and some more fruitful approaches to the question. The fact that Socrates' interlocutors are mathematicians, and that the conversation was recorded by a Megarian, has been suggested as one cause of the dialogue's failure to settle upon an answer. Approaches not very different from those taken in the dialogue are still discussed in modern epistemology.
The dialogue is split into roughly three sections: 1) Knowledge is perception; 2) Knowledge is true belief, and 3) Knowledge is justified true belief. It is preceded by a brief exchange between Euclides and Terpsion.