ensures that counting the same set in different ways can never result in different numbers (unless an error is made). This is the fundamental mathematical theorem that gives counting its purpose; however you count a (finite) set, the answer is the same. In a broader context, the theorem is an example of a theorem in the mathematical field of (finite) combinatorics--hence (finite) combinatorics is sometimes referred to as "the mathematics of counting".

Many sets that arise in mathematics do not allow