Socrates was the son of Sophroniscus, a sculptor. It is said that in early life he practiced his father's art. In middle life he married Xanthippe, who is legendary as a shrew, although the stories have little basis in ascertainable fact. It is not certain who were Socrates's teachers in philosophy, but he seems to have been acquainted with the doctrines of Parmenides, Heraclitus, Anaxagoras, and the atomists. He was widely known for his intellectual powers even before he was 40, when, according to Plato's report of Socrates's speech in the Apology, the oracle at Delphi pronounced him the wisest man in Greece. In that speech Socrates maintained that he was puzzled by this acclaim until he discovered that, while others professed knowledge without realizing their ignorance, he at least was aware of his own ignorance.

Socrates became convinced that his calling was to search for wisdom about right conduct by which he might guide the intellectual and moral improvement of the Athenians. Neglecting his own affairs, he spent his time discussing virtue, justice, and piety wherever his fellow citizens congregated. Some felt that he also neglected public duty, for he never sought public office, although he was famous for his courage in the military campaigns in which he served. In his self-appointed task as gadfly to the Athenians, Socrates made numerous enemies.

Aristophanes burlesqued Socrates in his play The Clouds and attributed to him some of the faults of the Sophists (professional teachers of rhetoric). Although Socrates in fact baited the Sophists, his other critics seem to have held a view similar to that of Aristophanes. In 399 B.C. he was brought to trial for corrupting youth and for religious heresies. Obscure political issues surrounded the trial, but it seems that Socrates was tried also for being the friend and teacher of Alcibiades and Critias, both of whom had betrayed Athens. The trial and death of Socrates, who was given poison hemlock to drink, are described with great dramatic power in the Apology, the Crito, and the Phaedo of Plato.

Socrates's contributions to philosophy were a new method of approaching knowledge, a conception of the soul as the seat both of normal waking consciousness and of moral character, and a sense of the universe as purposively mind-ordered. His method, called dialectic, consisted in examining statements by pursuing their implications, on the assumption that if a statement were true it could not lead to false consequences. The method may have been suggested by Zeno of Elea, but Socrates refined it and applied it to ethical problems.

His doctrine of the soul led him to the belief that all virtues converge into one, which is the good, or knowledge of one's true self and purposes through the course of a lifetime. Knowledge in turn depends on the nature or essence of things as they really are, for the underlying forms of things are more real than their experienced exemplifications. This conception leads to a teleological view of the world that all the forms participate in and lead to the highest form, the form of the good. Plato later elaborated this doctrine as central to his own philosophy. Socrates's view is often described as holding virtue and knowledge to be identical, so that no man knowingly does wrong. Since virtue is identical with knowledge, it can be taught, but not as a professional specialty as the Sophists had pretended to teach it. However, Socrates himself gave no final answer to how virtue can be learned.