On his return from a religious festival, Socrates encounters Polemarchus and returns with him to the house of his father, Cephalus, where the three men discuss justice. Both Cephalus and Polemarchus give traditional accounts of what justice is, which Socrates shows to be incomplete. Thrasymachus enters the debate, answering that the very conception of justice is a sham meant to keep the strong at bay. True justice, he contends, is the advantage of the stronger. Socrates tries to rebut Thrasymachus’s claim, but Thrasymachus remains unconvinced. Book I ends at this point, and the remaining nine books consist of Socrates working out, in dialogue with Glaucon and Adeimantus, a more robust definition of justice.

Glaucon and Adeimantus urge Socrates to prove that justice is good in itself and not only for its consequences. People act justly mostly out of fear of punishment, so if justice is not good in itself, and if they thought they could get away with it, people would have no reason not to act unjustly. Rather than answer their question directly, Socrates proposes to sketch out an ideal republic so they can determine what role justice plays in this republic.

Socrates proposes a principle of specialization, according to which each citizen has a particular role to play. A city needs producers, who produce food and shelter, as well as a class of guardians who protect the state’s interests. These guardians are raised according to a rigorous program of education that emphasizes physical fitness, honor, and wisdom. They are shielded from bad influences, such as myths that portray the gods as possessing vices, so that they don’t become brutish or soft. The best among the guardians are selected as rulers (also referred to as “guardians”), while the others become “auxiliaries,” who act as soldiers. To maintain this strict class structure of producers, auxiliaries, and guardians, Socrates invents a state-sanctioned mythology that discourages people from aspiring to a different class. Class mobility is only possible when a youth in one class is identified with abilities that clearly suit him for a different class.

Socrates identifies the four primary virtues in the different aspects of this republic: the guardians possess wisdom, the auxiliaries possess courage, and the whole possesses justice and moderation. Thus, the justice of an ideal republic does not reside in any particular part of the republic but rather in the structure of the republic as a whole.

Like the just city, the soul of a just person is divided into three parts, and the soul’s justice resides in the proper structuring of these parts. The soul has an appetitive part that desires money and other earthly goods, such as the producers; a spirited part that desires honor, such as the auxiliaries; and a rational part that desires truth, such as the guardians. The rational part rules in a just soul, ensuring the health of the whole.

The guardian class lives austerely, having no money or material possessions. They live communally, choose sexual partners by lot, and are separated from their children at birth so as to prevent family ties from overriding loyalty to the state. In a move that is revolutionary for its time, Socrates sees no reason why women should not have status equal to men.