In "metamathematics" in the 1920s, sets were divided into 2 kinds or types as an attempt to evade a certain paradox. ("The set of all men who do not shave thems

In "metamathematics" in the 1920s, sets were divided into 2 kinds or types as an attempt to evade a certain paradox. ("The set of all men who do not shave themselves" if I recall correctly.) One of those kinds of sets were called classes. So, nothing to do with OOP.