In books 2 through 4, as well as the other books of Plato's Republic, Socrates, Glaucon, and Adeimantus discuss the structure of a "city in speech, " or a theoretical, metaphorical city, in order to define and analyze the concept of justice. They propose two main variations of this city: the healthy city and the luxurious city.
The healthy city is also known as the true city or the city of utmost necessity. In this city, everyone's basic needs are met; the people have no unreasonable, luxurious, or greedy desires, and they provide for the community and the society according to their skills and abilities. The citizens produce the “right quality” and “right quantity” of goods and services in order to keep the city well-sustained.
Moderation is the foundation upon which this simple and, according to Socrates, nearly ideal city is built—the people are humble and comfortable, and they neither want nor need more than what they already have. Thus, this city is also a just city, as everyone knows their role and purpose within society; a just city is also "wise and courageous."
The other city is known as the luxurious or the feverish city, in which the people have both reasonable and unreasonable—but mostly luxurious or excessive—needs and desires; they can be greedy, materialistic, money-obsessed, and power-hungry. This city, therefore, cannot be just, and it essentially gives birth to injustice, war, conflict, and poverty, as well as the additional need for warriors, fighters, and guardians or protectors.
While this might not be the ideal city, it is more realistic than the healthy city, as there will always be people who will want more, be it out of necessity or luxury.