My dissertation on the Theaetetus is motivated by this question: Why is the text in which we expect to find Plato’s mature epistemology centered around relativism and what is Plato’s answer to it? The very posing of this question presupposes that relativism is the dialogue’s primary topic and demonstrating that this is so is one of my principal tasks. The first and longest part of the Theaetetus, which investigates the definition of knowledge as perception, is taken up in the critique of Protagoras, whose position is a relativist one: all judgments are true, there is no such thing as falsehood, disagreement is impossible, and everyone is self-sufficient with respect to knowledge. The Protagorean position stands for the idea that knowledge is incoherent and all activities that center around distinguishing true from false judgments, or the knower from the ignorant, are unintelligible. The Protagorean position is propped up by an account of perception, according to which perceptible reality is private to each perceiver and, indeed, to each perceptual episode. Generalized relativism follows on the assumption that all of our judgments are outcomes of such private perceptual episodes. The topic of relativism is ostensibly disposed of in the first part of the dialogue, where Socrates mounts a number of refutations that render the view absurd. And yet, I argue, despite the sustained critique of the Protagorean position, there are Protagorean commitments about human psychology that continue to grip Socrates and Theaetetus, the dialogue’s participants. Specifically, Socrates and Theaetetus, I argue, fail to do justice to their own insight that human psychic activity is teleologically organized with an aim to the truth. This prevents them from giving an account of falsehood and it blinds them to the fact that the distinction between the true and the false is one that human beings value. Consequently, the only theories of knowledge that Socrates and Theaetetus are in a position to consider are either retellings of Protagorean relativism or dangerously close to being such. In both cases, the fact that knowledge is valuable to human beings is obscured. Hence, I argue, the reason that the Theaetetus is centered around relativism is that its threat is pervasive and Plato’s answer is to remind us that it is up to us to stand up for the value of truth.