Protagoras (Πρωταγόρας; c. 490 – c. 420 BC) was a pre-Socratic Greek philosopher and rhetorical theorist. He is numbered as one of the sophists by Plato. In his dialogue Protagoras, Plato credits him with inventing the role of the professional sophist.

• Πάντων χρημάτων μέτρον ἄνθρωπον εἶναι, τῶν μὲν ὄντων, ὡς ἔστι, τῶν δὲ μὴ ὄντων, ὡς οὐκ ἔστιν.
  • Man is the measure of all things: of things which are, that they are, and of things which are not, that they are not.
  • Quoted in Plato, Theaetetus, sec. 152a. Translated by John Stuart Mill, "Plato", in the Edinburgh Review (April 1866)

• Δύο λόγους εἶναι περὶ παντὸς πράγματος.
  • There are two sides to every question.
  • Quoted in Diogenes Laërtius, Lives of Eminent Philosophers, b. 9, sec. 51. Translated by R. D. Hicks, Diogenes Laertius, vol. 2 (1925) p. 463. Similar "proverbial sayings" quoted by George Huntingford, Twelve Discourses on Different Subjects, vol. 2 (1797) p. 99, note: Παν πραγμα δυο εχει λαβας; Audi alteram partem.

• Περὶ μὲν θεῶν οὐκ ἔχω εἰδέναι οὔθ᾽ ὡς εἰσίν, οὔθ᾽ ὡς οὐκ εἰσίν· πολλὰ γὰρ τὰ κωλύοντα εἰδέναι, ἥ τ᾿ ἀδηλότης καὶ βραχὺς ὢν ὁ βίος τοῦ ἀνθρώπου.
  • As to the gods, I have no means of knowing either that they exist or that they do not exist. For many are the obstacles that impede knowledge, both the obscurity of the question and the shortness of human life.
  • Quoted in Diogenes Laërtius, Lives of Eminent Philosophers, b. 9, sec. 51. Translated by R. D. Hicks, vol. 2 (1925) p. 465. Diogenes adds: "For this introduction to his book the Athenians expelled him; and they burnt his works in the market-place, after sending round a herald to collect them from all who had copies in their possession."

• When they [the Athenians] meet for a consultation on civic art, where they should be guided throughout by justice and good sense, they naturally allow advice from everybody, since it is held that everyone should partake of this excellence, or else that states cannot be.
  • Quoted in Plato, Protagoras, sec. 322d–e. Translated by W. R. M. Lamb, Plato, vol. 4 (1924) p. 135

• You, Socrates, began by saying that virtue can't be taught, and now you are insisting on the opposite, trying to show that all things are knowledge, justice, soundness of mind, even courage, from which it would follow that virtue most certainly can be taught.
  • Quoted in Plato, Protagoras, sec. 361a–b. Translated by C. C. W. Taylor, Plato: 'Protagoras' (Oxford, 1976) p. 56

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  Encyclopedic article on Protagoras on Wikipedia
• "Protagoras", in Internet Encyclopedia of Philosophy’’
• "Protagoras", in Stanford Encyclopedia of Philosophy