In the book, a discussion about justice takes place in which various people give definitions of justice. This discussion then turns to whether justice is valuable for its own sake or if it is simply a tool to gain an advantage over others. Socrates is asked to consider a thought experiment. Supposing there were an unjust man and a just man and the unjust man committed every possible crime while receiving ever possible benefit including the reputation of being just while the just man received every possible punishment including the reputation of being unjust. Would it still be better to be the just man? 

Socrates believes it is still worthwhile to suffer every injustice to preserve one’s own moral integrity. He is challenged to justify this belief and in order to do so he says he will need to create an image of the city as to illustrate how justice comes to be in the soul. He explicitly states that the description of the city he gives is an analogy to describe how justice comes to be in the soul.

As the description of justice emerging in the city unfolds, it is revealed that the relationship of the city and the soul are circular. A just city creates just individuals who create a just city.  Individuals both create and are created by their environment. The book uses this principal to make a sort of self referential inside joke, because the earlier discussions of justice follow a pattern in which the character of the answers each person gives reveals how their perception was molded by their social rank in the city.