Natural Numbers:Postulates

Peano's Axioms for the Natural Numbers

1. 1 is a natural number.
2. For every natural number $n$, the successor to $n$ ($n+1$) is also a natural number.
3. 1 is not a successor to any natural number.
4. If two numbers $n_{1}$ and $n_{2}$ have the same successor, then $n_{1}=n_{2}$.
5. If a set $S$ contains 1, and also contains the successor of every element $n$ in $S$, then every natural number is in $S$.