Hippias, along with Protagoras and Prodicus, is one of the visiting Sophists whose presence in Athens causes Hippocrates's excitement at the beginning of this dialogue. The dates of Hippias's birth and death are unclear, but he was probably younger than Protagoras. He taught a range of subjects that included mathematics, history and science, but he was most famous for the exposition and criticism of works of literature. Plato indicates his rather disapproving opinion of Hippias's talents at line 347b, where Hippias offers to give a reading of a poem by Simonides that is discussed by Socrates and Protagoras; the others reject his offer.


Another Sophist, Prodicus was best known as a teacher of rhetoric, specializing in drawing fine distinctions between the meanings of words. In the Protagoras, Plato parodies this verbal quibbling (particularly in the passage 337a-c). Nonetheless, Socrates refers to himself as being a pupil of Prodicus, both in Protagoras (341a), and in some other dialogues.


Protagoras was the most famous Sophist of his day. Born around 485 BCE, he was renowned as a teacher of rhetoric and politics throughout Greece by the time of his death in 415 BCE. His most famous doctrine, that "man is the measure of all things," indicates that his views involved an early form of moral relativism. This is also a possible interpretation of his analysis of the concept of good (334a-c), where he asserts that a thing is good only in so far as it is good for something. Socrates's summary of his own position—that "all things are knowledge—justice, temperance, and courage" (361b)—closely follows the Greek syntax of Protagoras's famous doctrine, and must be understood as directly contending with it.

For Socrates, it is not man who measures all things, but knowledge that incorporates and structures all forms of the good. Despite this fundamental philosophical difference, Plato's representation of Protagoras is respectful when compared to his parodying of Hippias and Prodicus, or to his depiction of Sophists in other dialogues. If Protagoras holds false views, he is, nonetheless, a worthy interlocutor for Socrates, and allows Socrates to work out in detail some of his own arguments. Perhaps this explains why the dialogue is so inconclusive, and why many of Socrates's arguments are less than fully convincing.


Simonides was a renowned lyrical poet who was born some time around 556 BCE, and died in 468 BCE. The poem analyzed by Socrates and Protagoras survives only in the form of the quotations given in the Protagoras, but these allow a relatively complete reconstruction. It is cited at 339b, 339c, 344c-e, 345c-d and 346c-d.