Plato’s strategy in The Republic is to first analyze the primary notion of societal, or political, justice, and then to derive a comparable concept of individual justice. In Books II, III, and IV, Plato identifies political justice as harmony in a structured political body. An ideal society consists of three main classes of people: producers (craftsmen, farmers, artisans, and more), auxiliaries (warriors), and guardians (rulers); a society is just when relations between these three classes are right. Each group must perform its appropriate function, and only that function, and each must be in the right position of power in relation to the others. Rulers must rule, auxiliaries must uphold rulers’ convictions, and producers must limit themselves to exercising whatever skills nature granted them (farming, blacksmithing, painting, and more) Justice is a principle of specialization: a principle that requires that each person fulfill the societal role to which nature fitted him and not interfere in any other business.

In summary, they conclude justice is always accompanied by true pleasure. In all probability, none of these is actually supposed to serve as the main reason why justice is desirable. Instead, the desirability of justice is likely connected to the intimate relationship between the just life and the Forms he also theorized about. The just life is good in and of itself because it involves grasping these ultimate goods, and imitating their order and harmony, thus incorporating them into one’s own life. Justice is good, in other words, because it is connected to the greatest good, the Form of the Good.

Plato ends The Republic on a surprising note. Having defined justice and established it as the greatest good, he banishes poets from his city. Poets, he claims, appeal to the worst part of the soul by imitating unjust tendencies. By encouraging us to indulge ignoble emotions in sympathy with the characters we hear about, poetry encouraged us to indulge these emotions in life. Poetry, makes us unjust. Plato relates the myth of Er, which describes the trajectory of a soul after death. Just souls are rewarded for one thousand years, while unjust ones are punished for the same amount of time. Each soul then must choose its next life.