Aretē, meaning "excellence" or "virtue," is central to ancient Greek ethics, from the early poets through Plato and Aristotle to the Stoics. It is a quality necessary for success, and the aretai for moral success are moral virtues. Agathon, meaning "good," implies virtue when used to describe human beings, as does kalon (meaning "noble" or "beautiful"), the adjective most closely associated with aretē and nearly synonymous with agathon. Kakon implies the lack of virtue. In Hesiod and Solon the moral use of these terms is well established, and it is clearly prefigured in Homer. Virtue, to such poets, no less than to Plato, is long lasting and independent of wealth and power. The principal virtues under discussion before Socrates were shame (aidos), reverence (hosion), and justice (dike). Protagoras evidently considered shame and justice to be essential to a stable society.

Socrates and...