A definition that is harder to understand, but which is often used by mathematicians, is to say that a set is finite if there is no strict subset that can be put in 1-to-1 correspondence with the set itself. (A strict subset of a set is one that is not equal to the set itself). For example, the set of even numbers can be put in one-to-one correspondence with the set of natural numbers (2"n" corresponds to "n"), but the set of even numbers is also a strict subset of the natural numbers (there are many numbers which are not in the set of even numbers, but which are natural numbers, for example the number 1). This means that the set of natural numbers is infinite.