Theaetetus

Plato's fullest dialogue on the question 'What is knowledge?' It is framed as a conversation read aloud years later, after the young mathematician Theaetetus has been carried home wounded from battle. Socrates describes himself as a midwife who helps others give birth to ideas and tests whether they are sound. Theaetetus offers three answers. The first, that knowledge is perception, is linked to Protagoras' claim that man is the measure of all things and to the view that everything is in flux; Socrates argues that it undermines itself and that much of what we know is not perceived. A digression contrasts the philosopher with the man of the law courts. The second answer, that knowledge is true judgement, runs into the puzzle of how false judgement is possible, explored through the images of a wax tablet and an aviary. The third adds an account to true judgement. All three fail, and the dialogue ends unresolved.

It sets out the problems of epistemology with unusual care, and its third definition, true belief with an account, lies behind the modern analysis of knowledge and the debates it provoked.