Read Books III and IV of the Republic. Socrates presents his second form of the ideal city, comprised of three classes: rulers, guardians, and producers. He defines their respective roles, along with the qualities, education, and training appropriate to them. By the end of Book IV, Socrates proclaims the city just. By identifying justice at the level of the city, Socrates hopes to make an analogous definition of justice as a virtue at the personal level. Keep in mind that it is a long-standing point of contention among scholars as to Plato's intent behind presenting the Republic as a utopia.