After a religious festival, Socrates is invited to the house of a wealthy merchant named Cephalus. There, Socrates joins a discussion with Cephalus, Polemarchus, Glaucon, Adeimantus, and the Sophist Thrasymachus about the nature of justice. Socrates soon proves that Cephalus and Polemarchus' conception of justice as telling the truth and paying what is owed is insufficient, and he likewise disproves Thrasymachus's belief that justice is simply whatever is of most advantage to the stronger person or people. But Socrates does not state what his own idea of justice is. Instead, he proposes to "create" an ideal city that will show justice on a large scale. Once they have defined a just city, Socrates believes, they'll be able to examine justice in an individual.
Socrates' ideal city depends on education, specialization, and social structures that define family, behavior, and loyalty to the city. Each person will specialize in a specific occupation, an occupation that is chosen for them by the city based on their aptitudes and abilities as children. Education, especially of the guardians who will function as guards or soldiers as well as rulers, is the key to the success of the city. Imitative literature in which the author creates the voices of different characters, "imitating" human behavior, is forbidden. Literature must reflect only good behavior. Those who will be laborers or craftsmen will form the "producer" class. The best of the guardians are given special education to prepare them to rule. The others from the initial group of guardians will become the warriors for the city. Wives and children of the guardians are held in common. The rulers will lead very simple lives, forbidden to touch gold or silver or to own property. Their daily needs will be met by the other residents of the city so that guardians can rule without distraction.
Socrates turns to the question of who should rule the city. In support of his claim that the philosopher is the best ruler Socrates explains that the soul is made of three parts, the rational, the appetitive and the spirit. In the just man, each part of the soul performs its function, directed by reason, so that the appetites and spirit are controlled. Just as the rational part of the soul should rule over the others, the rational part of the city residents, the philosopher, should rule over the warriors and producers. This will require that philosophers become kings, or kings become philosophers. Only philosophers are able to truly love knowledge and truth, and only they recognize truth. Socrates presents the allegory of the cave. Imagine, he says, a cave, where men are chained in the dark and think that the shadows they see on the wall are reality, until one of them escapes into the sunlight and sees the physical world. The freed prisoner later returns and tries to teach the others about the nature of truth.
The philosopher is the best ruler because he understands that the objects of the physical world are copies, imitations, of the ideal Forms in the world of Ideas. The philosopher, because he understands the Forms, has greater understanding of everything. Consequently, the guardians must be educated in philosophy, as well as mathematics and logic. When the guardians are mature, their education includes the study of dialectic, the art of debate. Then, just as the prisoner returns to the cave, the guardians begin public service in preparation for later rule. Only the best of the guardians will become philosopher-kings. Socrates describes four kinds of cities, and the four kinds of people equivalent to the cities, ending with the worst, the tyrant. He ends with an examination of the tyrant, showing that the tyrant is neither just nor happy.
Socrates concludes with the myth of Er, a soldier who dies, but is returned to life and reports on the after life. He sees souls sorted out into those who were unjust, who must then suffer, and those who were just, who spend the afterlife in pleasure. At the end of their allotted time, souls are allowed to choose a new life. Socrates argues that the soul, since it can not be destroyed by death, or by evil, is immortal.
