Socrates now returns to the discussion that was interrupted at the beginning of the four bad kinds of people and the constitutions that correspond to them. By this examination, he hopes to finally compare the benefits of justice and injustice, and find out whether it is better to behave justly or unjustly.
There are four kinds of bad cities, and individual traits that go with them. These are, from least to most awful, the Timocracy, the Oligarchy, the Democracy, and the Tyranny; the Just City in decay passes through these stages in this order.
The first type of government is the Timocracy. This occurs when, inevitably, the guardians of the Just City make mistakes in the eugenics system, and mistaken births force them to put people with iron/bronze natures into positions of leadership. This doesn't change the structure of the city enormously--the guardians are still separate from the tradesmen, etc., but a few important and sinister changes take place. The mixed ruling class leads to a large group of leaders who are not philosophers, but are rather ruled by their spirited parts, and so are overly warlike and proud--thus the state is always at war. Also, some of these proud people start keeping private treasuries of money.
The transition from timarchy from oligarchy occurs because these leaders with their own treasuries start bending the laws relating to spending the money on themselves, until the constitution of the Just City is wholly destroyed, and gradually wealth becomes more important than virtue in measuring someone's worth. The oligarchic personality is defined by a disinterest in anything but money.
This leads to a total dissolution of the Just constitution for several reasons:.
1. Justice is gone because now those making lots of money-- traders, merchants, etc.--and the leaders are one and the same. The separation of roles that defined justice is gone.
2. The population is now wholly divided between rich and poor, with very different interests, whereas before it was unified.