Protagoras
On the usual reading (Plato’s Theaetetus): there is no truth beyond human perspectives – each person is the measure of what is true for them.
Placement
Sophist from Abdera who, according to Plato, was the first to call himself a sophist and to teach for a fee: man is the measure of all things.
Key ideas
- Homo mensura doctrine
- Relativism
- Rhetoric
Criticism
Criticized by 2
Socrates
criticised by
In Plato’s Protagoras, Socrates questions Protagoras’ claim to teach civic virtue and presses him on the unity of the virtues. Plato, Protagoras 319a–320c, 329c–334c, 349a–360e
Evidence: AI draft
Socrates › 470–399 BCE
Plato
criticised by
In the Theaetetus Plato argued that Protagoras’ ‘the human being is the measure of all things’ refutes itself, since it must concede that its opponents are right. Plato, Theaetetus 152a, 170e–171c; IEP “Protagoras”
Evidence: AI draft
Plato › 428–348 BCE
Position
Protagoras is best known for the statement “Man is the measure of all things, of the things that are, that they are, and of the things that are not, that they are not.” Read as relativism, it means that things are for each person as they appear to that person, so there is no standard of truth beyond human judgement. He was equally agnostic about the gods, saying that the obscurity of the subject and the shortness of life prevent knowledge. He claimed to teach good judgement in private and public affairs and to make the weaker argument the stronger. Plato’s Theaetetus examines his relativism at length.
Life & context
Protagoras of Abdera was the most famous of the sophists, itinerant teachers who offered paid instruction in rhetoric and civic excellence. He spent long periods in Athens, was on friendly terms with Pericles and reportedly drafted laws for the colony of Thurii. Plato made him the title figure of a dialogue. Only a few sentences of his writings survive.
Main works
- Truth (lost, c. 440 BCE)
- On the Gods (lost, c. 440 BCE)
As of November 2026 · Sources · Improve this page · How Agora is made