TheaetetusPlato

The Theaetetus asks what knowledge is and, remarkably for a Platonic dialogue, arrives at no positive answer. Three definitions are proposed and each is refuted.

First, Theaetetus suggests that knowledge is perception. Socrates connects this to Protagoras's doctrine that man is the measure of all things and to Heraclitean flux, then dismantles the combination: if everything is in flux and every perception is equally true, then no perception is more knowledgeable than any other, and the distinction between knowledge and ignorance collapses. Second, Theaetetus proposes that knowledge is true judgment. But Socrates asks how false judgment is possible at all. If you know something, you cannot mistake it for something else; if you do not know it, you have nothing to mistake. The puzzle of false belief occupies the center of the dialogue and anticipates problems that recur in modern epistemology. Third, Theaetetus offers knowledge as true judgment with an account (logos). But every attempt to specify what "account" means, whether it is an enumeration of elements, a distinguishing mark, or a statement of difference, either presupposes the knowledge it is supposed to explain or fails to add anything to true judgment alone.

The dialogue ends in aporia. Socrates compares himself to a midwife who helps others give birth to ideas but produces none of his own. The Theaetetus clears the ground: it shows what knowledge cannot be and thereby sets the terms for Plato's own positive theory in the Sophist and later dialogues.