Glaucon asks Socrates whether justice belongs 1) in the class of good things we choose to have for themselves, like joy, or 2) those we value for their consequences though they themselves are hard, like physical training, or 3) the things we value for themselves and their consequences, like knowledge. Socrates says justice is in the third and best group. Glaucon says that most people would say justice is valued not for itself but for its consequences, for justice is difficult, and thus often avoided.
Remember that Glaucon wants to be convinced that justice is a virtue, and that it is valued for itself as much as for its consequences—he is merely playing "devil's advocate" here.
Glaucon reviews Thrasymachus' arguments about justice. First, it is generally agreed that to do injustice is naturally good, but to suffer it, bad. Consequently men make laws, and what the laws require, they call just. The origin of justice is a compromise between right and wrong.
This is justice as a social contract, an agreement between people to avoid being unjust to each other so they may avoid being the victims of other people's injustice.
People value justice because they lack the power to do injustice. Justice is practiced only by compulsion, and for the good of others, since injustice is more rewarding than justice. Human nature inclines us towards injustice, but the law forces us to behave justly.
Justice lies in following the laws, whatever they may be; this is similar to the original definition given by Cephalus in Book I.
Glaucon tells the story of Gyges ring. A shepherd discovers a ring that makes its wearer invisible. The shepherd uses the ring to seduce the queen, murder the king and take the throne. If the power to do injustice were given to those who are usually too powerless to practice injustice, then, like the shepherd with the ring, they would be as unjust as others.
Through his story of Gyges' Ring, Glaucon contradicts the idea that laws equal justice. He argues that if a person could get away with injustice, as the shepherd does, he would behave unjustly.
Glaucon's brother Adeimantus says that it is merely the appearance of justice that is praised. An unjust person who has a reputation for justice leads a life of pleasure. The gods perceive truth and punish the unjust, but gods can be persuaded by prayers and sacrifices purchased by the unjust who have profited from their crimes.
Here the appearance of justice is seen as enough even for the gods, since they may be placated by other means.
Glaucon asks Socrates to describe what justice and injustice each do in themselves, how justice benefits those who have justice and how injustice harms them.
Glaucon and Adeimantus want Socrates to describe the pure qualities of justice and injustice.
Socrates proposes first to examine the justice of the city, because it is easier to determine what is just for the group then for the individual. He begins by specifying what the ideal city, the kallipolis, needs.
Socrates is proposing to argue from the general, the justice of the city or group, to the particular, the concept of justice and the individual.
A city needs people, food, shelter, and goods, with each person specializing in a particular occupation. The city needs merchants to trade with other cities, a marketplace, currency, local retailers, and people who perform manual labor for a wage. Luxury goods and services require a larger city, which leads to war to acquire more land. War requires an army, and soldiers require special skills.
Socrates examines the requirements of soldiers or "guardians." A guardian needs to be gentle to his own people, but harsh to others. Therefore the guardian must be a lover of learning, a philosopher, educated from childhood in music and poetry, then given physical training.
Notice that already Socrates emphasizes the importance of education and philosophy.
Poets, like Hesiod and Homer, tell inappropriate stories about gods committing impious actions, stories which might influence the citizens to act badly. Therefore, the city must only use stories depicting good behavior so as to influence the citizens of the city in positive ways.