In book 2 of Plato's Republic, Glaucon raises the question of whether justice is valued for itself or for its consequences. He believes that it is human nature to behave unjustly, since this is generally the easiest and most rewarding course, and that laws are made to force people to behave justly for the common good.

Glaucon illustrates his point that most people would behave unjustly if they had the chance to do so with the story of Gyges's ring. A shepherd finds a ring that makes him invisible, and uses it to infiltrate the royal court, seduce the queen, kill the king, and seize the kingdom. This, Glaucon asserts, is how people behave when they attain power. He then asks Socrates to demonstrate how justice benefits the just man.

Socrates replies that he will talk about cities rather than individuals, since it is easier to explain what is just on a collective level. A city needs to have citizens specializing in various activities to provide food, clothes, shelter, and the other necessities of life. It should be ruled by guardians who are philosophers, meaning that they love wisdom and knowledge. These guardians should receive a good general education, but moral instruction is paramount.