In early Plato, Socrates advances two theses regarding virtue. He suggests that virtue is a kind of knowledge, similar to the expertise involved in a craft; and he suggests that the five virtues (wisdom, temperance, courage, justice and piety) form a unity. There are two competing interpretations of Socratic virtue theory in the literature. The biconditional interpretation understands Socrates to hold that the virtues are 'distinct parts of a single whole,' each requiring a separate definition. On this interpretation, each virtue is knowledge of a different kind, and the virtues form a unity because they are inseparable---if one has possession of one of the virtues, she will necessarily possess them all. The identity interpretation understands Socrates to hold that the virtues are 'one and the same' thing, expert knowledge of good and bad.

On this interpretation, Socrates rejects the idea that the virtues are distinct parts of a whole, and the thesis that 'virtue is one' is taken quite literally. I provide an interpretation that accommodates both readings, and reconciles the (apparently conflicting) passages that give rise to them. On my interpretation, the primary question of the early dialogues (what is virtue?) serves to introduce two distinct searches, a conceptual search and a psychological search. There are times when Socrates seeks to clarify the concept of a virtue. There are other times when he seeks to identify the psychological state of being virtuous. For Socrates, the virtues are conceptually 'distinct parts of a whole,' but they are psychologically 'one and the same' expert knowledge. In my treatment of the Protagoras , I show that this knowledge engages one's most basic desire for living a good and happy life.

Virtue is an expertise concerned with measuring the value of things---measuring their impact on one's life as a whole. A close reading of the Gorgias reveals that this expertise is not wholly separate from the process of disciplining appetites. Undisciplined appetites lead to psychological deterioration, which means that the undisciplined agent will eventually become psychologically incapable of developing the expertise for living a good and happy life.