The life and teachings of Socrates (c. 469-399 B.C.) 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 B.C.) 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 come from his contemporaries. These accounts are mixed and often biased by the interpretations the author wishes to place on Socrates.
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, we know that he did serve as a hoplite (heavy infantry) in several battles during the Peloponnesian War and that he was distinguished for his bravery. In 399, 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. Socrates' response to the charges brought against him are recorded by Plato in The Apology.
Plato (c. 427-347 B.C.) was one of Socrates' greatest admirers, and our knowledge of Socrates stems mostly from Plato's dialogues (for other accounts, see Aristophanes' satirical presentation in The Clouds and the writings of Xenophon). Plato was born into a prominent Athenian family, and would have been expected to pursue a career in politics. However, the short-lived Spartan- imposed oligarchy of the Thirty Tyrants (404-403) and the trial and execution of his mentor Socrates led Plato to become disgusted with the Athenian political life, and he devoted himself instead to teaching and philosophical inquiry. To that end, he founded the Academy around 385 B.C., which counted Aristotle among its students. The Academy lasted in some form until 527 A.D., 912 years in total, and has 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 small literary genre at the time: not just Plato, but many of Socrates' other students recorded philosophical debates in this form. Though we can't be certain as to the specific dates of composition, Plato's dialogues can generally be 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 these 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. The Euthyphro is one of Plato's earlier dialogues, in which we find none of his more characteristic doctrines, but rather an attempt to present Socrates the teacher. Instead of positive doctrines or ideas, the dialogue is characterized by the use of Socratic irony in an attempt to teach others to recognize their own ignorance.
Socrates is treating Euthyphro as the teacher when in fact Socrates is teaching Euthyphro
Plato suggesting that there is no such thing as a definition of holiness, that there is no one feature that all holy deeds have in common?
What Plato/Socrates is challenging is Euthyphro's/everyone's knowledge or assumed knowledge of anything, not the can we know anything idea, but have we challenged our beliefs? Are we sure that the conclusion we hold is conclusion enough? Peirce and James pick this up again a few years later.