Cardinal of Finite Ordinal

Theorem
Let $n$ be a finite ordinal.

Let $\left|{n}\right|$ denote the cardinal number of $n$.

Then:


 * $\left|{n}\right| = n$

Proof
Since $n$ is an ordinal, it follows that $\left|{n}\right| \le n$ by Cardinal Number Less than Ordinal: Corollary.

Hence, $\left|{n}\right|$ is also a finite ordinal.

Since $n$ is an ordinal, it also follows that $n \sim \left|{n}\right|$ by Ordinal Number Equivalent to Cardinal Number.

By Equality of Natural Numbers, it follows that $n = \left|{n}\right|$.