The first book, two definitions of justice are proposed but deemed inadequate. Returning debts owed, and helping friends while harming enemies are common sense definitions of justice that, Socrates shows, are inadequate in exceptional situations, and thus lack the rigidity demanded of a definition. Yet he does not completely reject them for each expresses a common sense notion of justice which Socrates will incorporate into his discussion of the just regime in books II through V. At the end of Book I, Socrates agrees with Polemarchus that justice includes helping friends, but says the just man would never do harm to anybody. Thrasymachus believes that Socrates has done the men present an injustice by saying this and attacks his character and reputation in front of the group, partly because he suspects that Socrates himself does not even believe harming enemies is unjust. Thrasymachus gives his understanding of justice and injustice as "justice is what is advantageous to the stronger, while injustice is to one's own profit and advantage". Socrates finds this definition unclear and begins to question Thrasymachus. Socrates then asks whether the ruler who makes a mistake by making a law that lessens their well-being, is still a ruler according to that definition. Thrasymachus agrees that no true ruler would make such an error. This agreement allows Socrates to undermine Thrasymachus' strict definition of justice by comparing rulers to people of various professions. Thrasymachus consents to Socrates' assertion that an artist is someone who does his job well, and is a knower of some art, which allows him to complete the job well. In so doing Socrates gets Thrasymachus to admit that rulers who enact a law that does not benefit them firstly, are in the precise sense not rulers. Thrasymachus gives up, and is silent from then on. Socrates has trapped Thrasymachus into admitting the strong man who makes a mistake is not the strong man in the precise sense, and that some type of knowledge is required to rule perfectly. However, it is far from a satisfactory definition of justice.
At the beginning of Book II, Plato's two brothers challenge Socrates to define justice in the man, and unlike the rather short and simple definitions offered in Book I, their views of justice are presented in two independent speeches. Glaucon's speech reprises Thrasymachus' idea of justice; it starts with the legend of Gyres who discovered a ring that gave him the power to become invisible. Glaucon uses this story to argue that no man would be just if he had the opportunity of doing injustice with impunity. With the power to become invisible, Gyges is able to seduce the queen, murder the king, and take over the kingdom. Glaucon argues that the just as well as the unjust man would do the same if they had the power to get away with injustice exempt from punishment. The only reason that men are just and praise justice is out of fear of being punished for injustice. The law is a product of compromise between individuals who agree not to do injustice to others if others will not do injustice to them. Glaucon says that if people had the power to do injustice without fear of punishment, they would not enter into such an agreement. Glaucon uses this argument to challenge Socrates to defend the position that the just life is better than the unjust life. Adeimantus adds to Glaucon's speech the charge that men are only just for the results that justice brings one- fortune, honor, reputation. Adeimantus challenges Socrates to prove that being just is worth something in and of itself, not only as a means to an end.
Glaucon's speech seduces Socrates for it is in itself contradictory. Glaucon has openly, passionately and forcibly argued for the superiority of the unjust life, something truly unjust men would never do in public. Socrates says that there is no better topic to debate. In response to the two views of injustice and justice presented by Glaucon and Adeimantus, he claims incompetence, but feels it would be impious to leave justice in such doubt. Thus the Republic sets out to define justice. Given the difficulty of this task as proven in Book I, Socrates in Book II leads his interlocutors into a discussion of justice in the city, which Socrates suggests may help them see justice not only in the person, but on a larger scale, "first in cities searching for what it is; then thusly we could examine also in some individual, examining the likeness of the bigger in the idea of the littler.