But if these are the only reasons for praising virtue...

But if these are the only reasons for praising virtue, how can justice in itself and apart from any rewards, be more profitable than injustice? Plato’s answer is to try to show what justice is, first in the state, and then in the soul. He outlines a state in which all basic needs are met. Three classes of citizen are required: artisans and laborers to produce the material needs of society; soldiers to defend the state; and rulers to organize its social life.

The key transition here is from recognizing three functions which have to be discharged in social life to asserting that three distinct and separate classes of citizens are needed, one to discharge each function. Plato relies for this transition upon two beliefs, one not certainly true, and one…