He argued that believing that others' opinions about the world are valid and must be respected, even if our own experience of truth is different, is necessary for a community to base itself and its decisions on open, democratic debate.

[1][4] According to Aulus Gellius, he originally made his living as a porter, but one day he was seen by the philosopher Democritus carrying a load of small pieces of wood he had tied with a short cord.

[7][8] His death, then, may be presumed to have occurred circa 420 BC, but is not known for certain, since assumptions about it are based on an apparently fake story about his trial for asebeia (impiety) in Athens.

[9] Plutarch wrote that Pericles and Protagoras spent a whole day discussing an interesting point of legal responsibility, that probably involved a more philosophical question of causation:[10] "In an athletic contest, a man had been accidentally hit and killed with a javelin.

He especially was involved in the question of whether virtue could be taught, a commonplace issue of fifth century BC Greece, that has been related to modern readers through Plato's dialogue.

In his eponymous Platonic dialogue, Protagoras interprets a poem by Simonides, focusing on the use of words, their literal meaning, and the author's original intent.

[13] Diogenes Laërtius reports that Protagoras devised a taxonomy of speech acts, such as assertion, question, answer, command, etc.

[14][15] The titles of his books, such as Technique of Eristics (Technē Eristikōn, literally "Practice of Wranglings"— wrestling used as a metaphor for intellectual debate), prove that Protagoras also was a teacher of rhetoric and argumentation.

[14] Eusebius quoting Aristocles of Messene says that Protagoras was a member of a line of philosophy that began with Xenophanes and culminated in Pyrrhonism.

[13] Plato ascribes relativism to Protagoras and uses his character Socrates as a foil for his own commitment to objective and transcendent realities and values.

"[25][26][27] According to Diogenes Laërtius, the outspoken, agnostic position taken by Protagoras aroused anger, causing the Athenians to expel him from the city, and all copies of his book were collected and burned in the marketplace.

[28] The classicist John Burnet doubts this account, however, as both Diogenes Laërtius and Cicero wrote hundreds of years later and as no such persecution of Protagoras is mentioned by contemporaries who make extensive references to this philosopher.

According to Diogenes Laërtius other books by Protagoras include: On the Gods, Art of Eristics, Imperative, On Ambition, On Incorrect Human Actions, On Those in Hades, On Sciences, On Virtues, On the Original State of Things and Trial over a Fee.