2 types of classes

There are 2 types of classes: proper classes and improper classes.

Proper classes only include members that share the same underlying cause for existing. "Birds" for example is a proper class because it only includes animals that share a common ancestor.

Improper classes include many different members with widely varying reasons for existing. "People wearing red shirts" for example would be an improper class. People wear red shirts for many different reason and sometimes for no reason at all.