Plato’s Theaetetus:An Inquiry into Knowledge
Richard Polt, who has taught philosophy at Xavier University for over thirty years, leads a lively five-lecture exploration of one of Plato's most fascinating dialogues: the Theaetetus. The course takes up a fundamental question that has occupied thinkers for millennia. What is knowledge? Through careful reading and spontaneous discussion, Polt shows how Plato dramatizes this question in a conversation between Socrates, the young mathematician Theaetetus, and his teacher Theodorus, weaving together rigorous argument and vivid human personality.
Theaetetus’s first proposal is that knowledge is simply perception. Socrates quickly connects this idea to the relativism of Protagoras, who claimed that each person is the measure of all things, and to the radical flux theory of Heraclitus, who held that everything is constantly changing. Together these doctrines form a powerful but unsettling picture of reality in which there are no stable truths, only momentary appearances. Polt traces how Socrates builds up this theory with surprising generosity before turning to dismantle it.
The middle lectures examine a series of arguments against relativism, including the famous self-refutation argument: if everyone's perception is equally true, then those who reject relativism are also right, and relativism undermines itself. Socrates also challenges the theory through questions about expertise and prediction, asking whether some people are genuinely wiser than others. A memorable digression contrasts the philosopher with the lawyer, raising the question of what philosophy itself really is.
The decisive blow against knowledge as perception comes when Socrates argues that, while the eyes and ears are instruments through which we perceive, it is the soul that unifies sensory information and grasps concepts like existence, sameness, and difference. These concepts cannot be reduced to any single sense, and without them there is no knowledge. This insight shifts the inquiry from perception to judgment, and the final lectures explore whether knowledge might be true judgment, or true judgment combined with an account or explanation.
Each proposed definition meets its own difficulties, and the dialogue ends without a final answer. Polt shows why this apparent failure is itself a philosophical achievement: the conversation has cleared away false assumptions, sharpened the participants' thinking, and left them better equipped to continue the search. The course is designed as an accessible introduction to Plato, to the Theaetetus, and to the practice of philosophical inquiry itself. No prior background in philosophy is needed.
You can watch the lectures below, browse the chapter index, or watch on YouTube.
Richard Polt
Richard Polt is a Professor of Philosophy at Xavier University. He holds a B.A. in philosophy from the University of California at Berkeley and a Ph.D. from the Committee on Social Thought at the University of Chicago. His interests include German and Greek philosophy, and he has taught courses on Heidegger, memory, history, technology, and African-American philosophy.
Polt's publications include Heidegger: An Introduction, The Emergency of Being: On Heidegger's Contributions to Philosophy, and Time and Trauma: Thinking Through Heidegger in the Thirties. He has also edited or co-edited volumes including A Companion to Heidegger's Introduction to Metaphysics, Heidegger's Being and Time: Critical Essays, After Heidegger?, and The Task of Philosophy in the Anthropocene.
