Al-Farabi was a famous philosopher from the Islamic philosophy tradition. On the question of why and how societies decline from the ideal, he said that this happens for one of the three reasons:
- Because of Ignorance
- Because of Wickedness
- Because of Error
Ignorant cities fail to grasp the true nature of humanity and the reason for its existence.
Why because of Wickedness:
Wicked cities once new or perhaps still know, what is the reason for humanity's existence, but they fail to act on that knowledge. The wicked ones fail to act on knowledge because they are wicked. That is how such societies reach downhill from the pinnacle of their existence.
Why because of Error:
The errant societies fail to act on the knowledge of true nature of humanity's existence and the reason for its existence. This happens because either they misapply their knowledge or their rulers mislead them.
Source: The History of Philosophy by A. C. Grayling