Plato: Theatetus, avoid Relativism; "Man is not the measure of all things."

In Theaetetus, Plato mounts a profound critique of relativism—not only by exposing its internal contradictions, but by showing that the very attempt to articulate it presupposes a stability that relativism itself denies. Socrates reveals that in order to assert that “everything is flux” or “man is the measure of all things,” one must already rely on stable categories such as truth, identity, and intelligibility. In other words, to deny these is to simultaneously affirm them.

Join us. Here is a link to the reading material. It is a word doc with the important verses and commentary.

https://docs.google.com/document/d/1wOs3ayzagGmd4xpi8yy0mAna_ILTJH-8/edit?usp=drive_link&ouid=117509398413484629402&rtpof=true&sd=true

1. The retorsion argument, which Socrates levels against Protagoras’ dictum, shows that relativism refutes itself. If all beliefs are equally true, then the belief that relativism is false must also be true—at least for those who reject relativism. This results in the collapse of relativism's claim to universal validity. “If what each man judges is true for him, then the claim that this is not so must be true for those who believe it isn't.” (171a–172b)

2. The very act of asserting relativism requires stepping outside of it. In using reasoned discourse (logos), one appeals to shared standards of truth and coherence that relativism cannot account for. The moment the relativist tries to defend his position, he abandons it. “A man must be able to give an account (logos) of what he knows, otherwise he cannot be said to know it.”(183b–184a)

3. Plato underscores that language, meaning, and knowledge presuppose a stable subject—a unified self that persists through time. This unity is incompatible with a doctrine of pure flux. Even to speak or to think requires some constancy of identity. The Heraclitean theory, when taken seriously, destroys the possibility of discourse and renders the very notion of a self-incoherent.

4. If our material brain is constantly changing, can we say that it persists in time? Would a non-material being be a better explanation of “me” as “me”. I am a someone not a something.

5. The consequences of this position are existential as well as epistemological. If all truth is relative, dialogue collapses. Without dialogue, the soul is isolated, and philosophy—understood as the shared pursuit of truth—is impossible. Plato thus shows that the very possibility of soul, knowledge, and community depend on the rejection of radical relativism and flux.

“Without truth, there is no dialogue; without dialogue, there is no polis, no philosophy, and no soul in the Platonic sense.” (implicit throughout; cf. Republic and Phaedrus)

6. The aporetic ending of Theaetetus does not signal defeat but rather clears the ground for a more adequate account of knowledge—one that will ultimately require a metaphysical foundation in the Forms. Plato demonstrates that relativism is not merely mistaken, but unintelligible on its own terms. Any coherent theory of knowledge must assume the very conditions relativism seeks to deny.

7. There’s no more fundamental division in philosophy than the one between those who accept this argument and those who do not. It runs deeper than familiar oppositions—deeper even than the divide between theists and atheists, or between materialists and those who believe in some kind of non-material reality.

8. Why? Because those debates already presuppose something crucial: that there is such a thing as truth, that something is the case, and that the disagreement is about what is the case. Both sides—say, the theist and the atheist—are operating within the shared framework that there is a truth to be known, and they argue by trying to show that their view better captures it. That’s what makes a real argument possible, rather than just a quarrel. Each side takes up a position.

9. An opponent who refuses to take up any position isn't really participating in the debate. If someone like Protagoras says, “Man is the measure of all things,” he’s making a claim. But if he retreats and says merely, “Well, that’s just how it seems to me,” then he's no longer stating a theory. He’s evading the conversation entirely. Without a claim, there’s nothing to respond to.

10. In that sense, debates like theism vs. atheism or materialism vs. non-materialism are meaningful precisely because both sides are engaged in the pursuit of truth. But in this deeper divide—the one I began with—even that shared presupposition is under question. Is anything the case at all? Is there any truth to seek? And that’s why it’s not even clear whether a real debate can occur between those who accept the argument and those who fundamentally reject it. They don’t share enough ground to even begin.

11. That’s what makes this divide so profound. It’s not just about what is true—it’s about whether truth itself can even be acknowledged.

12. Your instinct is right that this critique of relativism is not just logical but existential. If truth cannot be shared, the soul is cut off from others. Plato’s concern here is deeply ethical and political: without truth, there is no dialogue; without dialogue, there is no polis, no philosophy, and no soul in the Platonic sense. Society breaks down the inner cohesion of self is broken.

13. To put it dramatically: truth is the condition of being a person of being a soul. Consider Helen Keller.