Natural Number is Ordinal/Proof 2
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Let $n \in \N$ be a natural number.
Then $n$ is an ordinal.
From Superinductive Class under Successor Mapping contains All Ordinals, it follows by the Principle of Mathematical Induction that every natural number is an ordinal.
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $5$: Ordinal Numbers: $\S 1$ Ordinal numbers: Theorem $1.5$