Protagoras really has two authors: Socrates, the main speaker, and Plato, his pupil. Plato never appears in the dialogue, but his shaping hand is always present. Socrates engages in dialogues with other characters—with Protagoras, with Prodicus, and so on—but these dialogues bear traces of another dialogue taking place in this text: that between Plato and his teacher, Socrates. This dialogue between Plato and Socrates is generally (and quite accurately) represented as a crucial founding point of Western philosophy. Socrates left no writings, and we can only approach him through the writings of others. Plato wrote a great deal, but his writing generally takes the form of conversations between Socrates and others; Plato himself is absent.

Socrates (c. 469-399 BCE)

The life and teachings of Socrates stand at the foundation of Western philosophy. He lived in Athens during a time of transition (Athens' defeat at the hands of Sparta in the Peloponnesian War (431—404 BCE) ended the Golden Age of Athenian civilization) and had a tremendous influence on the Athenian youth of his day. Socrates himself never recorded his thoughts, so our only record of his life and thought comes from his contemporaries. These accounts are mixed and often biased by the authors' personal interpretations.

It seems that Socrates led a very simple life, renouncing wealth and holding himself aloof from political ambitions, preferring instead to mingle with the crowds in Athens' public places, engaging whomever he could in conversation. Nonetheless, he did serve as a hoplite (heavy infantryman) in several battles during the Peloponnesian War, and that he was distinguished by his bravery. In 399 BCE, Socrates was brought before a jury of around 500 Athenians on charges of not recognizing the gods recognized by the state, of inventing new deities, and of corrupting the youth of Athens.

The most likely reason for this trial is Socrates' close association with a number of men who had fallen out of political favor in Athens, but because an amnesty had been declared for political offenders, other charges had to be brought against him. Socrates was found guilty by a narrow margin and then sentenced to death.

Plato (c. 427-347 BCE)

Plato, the author of The Apology, which describes Socrates's trial, was one of his greatest admirers, and our knowledge of Socrates stems mostly from Plato's dialogues. (For competing accounts, see Aristophanes' satirical presentation in The Clouds and the writings of Xenophon.) Plato was born into a prominent Athenian family, and was expected to pursue a career in politics. However, the short-lived Spartan-imposed oligarchy of the Thirty Tyrants (404—403 BCE) and the trial and execution of his mentor, Socrates, led Plato to become disgusted with Athenian political life, and he devoted himself instead to teaching and philosophical inquiry. To that end, he founded the Academy around 385 BCE, which counted Aristotle among its students. The Academy lasted in one form or another until 527 CE, and served as the prototype for the Western university system.

Plato's thought is mostly recorded in the form of dialogues that feature Socrates as the protagonist. Apparently, the Socratic dialogue was a genre form at the time: not just Plato, but many of Socrates' other students recorded philosophical debates in this form. Plato's dialogues are generally classed into early, middle, and late periods. The early dialogues were written soon after Socrates' death, and in them we get the clearest picture of Socrates and Socratic philosophy. As Plato matured, however, he developed an increasingly distinct voice and philosophical outlook.

The figure of Socrates in the middle and late dialogues (The Republic and Phaedo are two exemplary works of the more mature Plato) becomes more of a mouthpiece for Plato's own views. Meno is generally considered one of Plato's earlier dialogues, with the conversation dateable to about 402 BCE.