natural numbers are well-ordered
In many proofs, one needs the following property of positive and nonnegative integers:
Theorem. Any non-empty set of natural numbers contains a least number.
Proof. Let be an arbitrary non-empty subset of . Denote
Then of course, . There exists surely an element of such that , since otherwise the induction property would imply that . Because , there is a number of the set such that . On the other , we must have . Consequently, and therefore
Hence, has the least number . Q.E.D.