Purakau: Turanganui-a-kiwa

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 (c.427-347 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 is not (Theaetetus 210c; cp. 183a5, 187a1).
This matters, given the place that the Theaetetus is normally assigned in the chronology of Plato’s writings. Most scholars agree that Plato’s first writings were the “Socratic” dialogues (as they are often called), which ask questions of the “What is…?” form and typically fail to find answers: “What is courage?” (Laches), “What is self-control?” (Charmides), “What is justice?” (Alcibiades I; Republic 1), “What is holiness?” (Euthyphro), “What is friendship?” (Lysis), “What is virtue?” (Meno), “What is nobility?” (Hippias Major). After some transitional works (Protagoras, Gorgias, Cratylus, Euthydemus) comes a series of dialogues in which Plato writes to a less tightly-defined format, not always focusing on a “What is…?” question, nor using the question-and-answer interrogative method that he himself depicts as strictly Socratic: the Phaedo, the Phaedrus, the Symposium, and the Republic. In these dialogues Plato shows a much greater willingness to put positive and ambitious metaphysical views in Socrates’ mouth, and to make Socrates the spokesman for what we call “Plato’s theory of Forms.”
After these, it is normally supposed that Plato’s next two works were the Parmenides and the Theaetetus, probably in that order. If so, and if we take as seriously as Plato seems to the important criticisms of the theory of Forms that are made in the Parmenides, then the significance of the Theaetetus’s return to the aporetic method looks obvious. Apparently Plato has abandoned the certainties of his middle-period works, such as the theory of Forms, and returned to the almost-sceptical manner of the early dialogues. In the Theaetetus, the Forms that so dominated the Republic’s discussions of epistemology are hardly mentioned at all. A good understanding of the dialogue must make sense of this fact.