Justice

Justice is the order that lets unequal claims live together — render to each what each is due.

Before justice is a virtue of persons it is a problem of the city: when your good and mine collide, by what measure do we settle it? Thrasymachus answers cynically — justice is whatever serves the strong. Plato spends the rest of the Republic refusing that answer, arguing that justice is a harmony, in the soul and in the state, where each part does its own work and none usurps another's.

It opens the conversation that runs through Aristotle's fairness, the social-contract thinkers, and every modern argument about rights and the common good.