The Theaetetus, which probably dates from about 369 BC, is arguably Plato's greatest work on epistemology. (Arguably, it is his greatest work on anything.) Plato (427-367 BC) has much to say about the nature of knowledge elsewhere. But only the Theaetetus offers a set-piece discussion of the question “What is knowledge?”
Like many other Platonic dialogues, the Theaetetus is dominated by question-and-answer exchanges, with Socrates as main questioner. His two respondents are Theaetetus, a brilliant young mathematician, and Theaetetus' tutor Theodorus, who is rather less young (and rather less brilliant).
Also like other Platonic dialogues, the main discussion of the Theaetetus is set within a framing conversation (142a-143c) between Eucleides and Terpsion (cp. Phaedo 59c). This frame may be meant as a dedication of the work to the memory of the man Theaetetus. Sedley 2004: 6-8 has argued that it is meant to set some distance between Plato's authorial voice and the various other voices (including Socrates') that are heard in the dialogue. Alternatively, or also, it may be intended, like Symposium 172-3, to prompt questions about the reliability of knowledge based on testimony. (Cp. the law-court passage (Theaetetus 201a-c), and Socrates' dream (Theaetetus 201c-202c).)
The Theaetetus’ most important similarity to other Platonic dialogues is that it is aporetic—it is a dialogue that ends in an impasse. The Theaetetus reviews three definitions of knowledge in turn; plus, in a preliminary discussion, one would-be definition which, it is said, does not really count. Each of these proposals is rejected, and no alternative is explicitly offered. Thus we complete the dialogue without discovering what knowledge is. We discover only three things that knowledge ...