[[category:There are several definitions of a class in use. The following definition is given by Copi. [1]
An ordered 4-tuple (C,(∩,∪),~,(1,0)) is called a class if it satisfies the following axioms.
1. (C, ∩, 1) is an abelian monoid
2. (C, ∪, 0) is an abelian monoid
3. 0 ≠ 1
4. for every a in C,
5. and for every a, b, and c in C,