A substantial part of the world's population belongs to neither group, but forms another and very important segment—"the middle class". Property is the principal interest of the middle class; this has tended to make it conservative. Important as it is for the development of political groups, the middle class is not as indispensable as the people and the leaders are. Identification with property gives the middle class its basic character as well as its importance throughout the history of the human society. Like the leadership class, the middle class is a product of economic opportunity. If economic opportunity promoted the origin and early growth of the middle class, political opportunity helped it to maturity, power, and influence. The middle class develops because of expanding economic opportunity engendered by political organizations. Its decline concurs with their decline. The end of the Greek city-states marked the end of the economic importance of the middle class, but not the end of its existence.