Plato vs Protagoras
"Man is the measure" against the philosophical defence of objective truth
Venue: Plato, *Protagoras* (c. 390 BC); *Theaetetus* (c. 369 BC).
The founding confrontation between philosophical realism and sophistic relativism.
Plato's dialogues *Protagoras* and *Theaetetus* stage philosophy's first sustained confrontation with relativism. Protagoras's famous dictum — "man is the measure of all things, of those that are that they are, and of those that are not that they are not" — is given its most careful refutation in the *Theaetetus*: if all opinions are equally true relative to their holders, then Protagoras's own theory is not specially privileged, and the moves required to maintain it are self-undermining. The *Protagoras* dialogue further examines whether virtue is teachable, with Socrates pressing the sophist on whether what he charges to teach can coherently be conceived. The exchange — asymmetric, since Plato writes both sides — is the founding moment of the philosophical critique of relativism and a permanent reference point.
Historical Context
Protagoras (c. 490–420 BC) was the most famous of the early sophists, a professional teacher of rhetoric and statecraft. The dramatic setting of Plato's *Protagoras* is around 432 BC; Plato wrote the dialogue some forty years later, after Socrates's death and the political collapse of Athens.
Parties
Truth and virtue are objective, knowable, and unitary; sophistic relativism is self-undermining and morally corrosive. Genuine teaching requires that what is taught be a real, knowable subject.
Key arguments
- Self-refutation: if all opinions are true relative to their holders, the opinion that some opinions are false is also true relative to its holder — and so is its negation, undermining the very notion of "true" the relativist needs.
- Virtue is teachable only if it is one knowable subject; Protagoras's claim to teach virtue presupposes more about virtue than his relativism allows.
- The doctrine of Forms (developed in middle dialogues) supplies the metaphysical realism the refutation of Protagoras presupposes.
- Moral seriousness: relativism in ethics dissolves the binding force of any moral claim, including the sophists' own implicit claim to be teaching something worth paying for.
Man is the measure of all things: each person's perceptions and judgements are true for them, with no further standpoint from which to call them simply true or false. Excellence (*aretē*) can be taught as social-political skill, not as access to a Form.
Key arguments
- Phenomena are relative to perceivers; what is hot to one is cold to another, and there is no view from nowhere that adjudicates.
- "Of all things the measure is man, of things that are that they are, and of things that are not that they are not."
- Virtue is teachable in the sense of practical skill in civic life; Plato's demand for a metaphysical theory of virtue confuses pedagogy with metaphysics.
- Pragmatic refinement (in the *Theaetetus* extension): some opinions are better than others, not by being more "true" absolutely but by being more useful, healthier, more conducive to flourishing.
Allied schools
Dimensions Engaged
Observer
Observer · Knowledge Extent: is there objective truth accessible to philosophical inquiry, or are all judgements perspective-relative?
Information
Bears on Information · Ontological Status: are truth-bearers properties of propositions in themselves, or relations between propositions and observers?
Verdict in retrospect
Plato's critique of relativism — especially the self-refutation argument — remains a standard reference point and is widely held to refute strong global relativism. Sophisticated forms of relativism (cultural, conceptual, framework) survive and continue to be philosophically defended. The deeper question of how to ground objectivity without Platonic Forms remains live; Protagoras's pragmatic-relativist line, refined, has substantial 20th-century echoes (James, Rorty).
Related Debates
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Further reading
- Plato, *Protagoras* (tr. Lombardo & Bell, 1992); *Theaetetus* (tr. Burnyeat & Levett, 1990)
- Burnyeat, "Protagoras and Self-Refutation in Plato's Theaetetus" (1976)
- Lee, *Epistemology after Protagoras* (2005)