The dialogue begins when the mathematician Theodorus introduces his brilliant young student, Theaetetus, to Socrates. Eager to discover the true nature of knowledge, Socrates engages the youth in a series of probing examinations. Theaetetus first proposes that knowledge is simply sensible perception, aligning himself with the famous assertion of Protagoras that 'man is the measure of all things.' Socrates subjects this view to an extensive critique, connecting it to the Heraclitean doctrine that all things are in a state of perpetual flux. He demonstrates that if everything is constantly changing, no stable definition or true knowledge can ever be attained through mere sensation.
After refuting the identification of knowledge with perception, Socrates guides Theaetetus toward a second definition: that knowledge is true opinion. This leads the interlocutors into a perplexing puzzle concerning the nature of error. If a person thinks, they must either know or not know the object of their thought; therefore, how is false opinion even possible? Socrates attempts to resolve this by introducing the famous metaphor of the mind as a block of wax, where memories and perceptions are imprinted like seals. Yet even this waxen tablet model proves inadequate to explain errors, particularly when mistakes occur purely within the realm of mental concepts, such as in mathematical calculations.
Faced with the failure of the waxen tablet hypothesis, the discussion turns to a third and final proposal: that knowledge consists of true opinion combined with definition or rational explanation. Socrates tests various interpretations of what an explanation might mean, ranging from the mere expression of thought in speech to the enumeration of elementary parts and the perception of a unique characteristic mark. Each attempt breaks down under close dialectical scrutiny, revealing the internal contradictions and insufficiency of the proposed definitions.
Throughout the work, Plato weaves in famous digressions, most notably a striking contrast between the petty, hurried life of the practical lawyer and the leisurely, sublime existence of the true philosopher who contemplates absolute justice and the nature of the universe. Although the dialogue ultimately concludes without reaching a definitive, positive answer to the primary question, its negative method clears away groundless assumptions and confusion. Socrates gently concludes the session, reminding them that the exercise has helped purge false conceit, leaving the minds of the participants better prepared for genuine philosophical enquiry.
