Cardinal Number is Ordinal

Theorem
Let $S$ be a set such that $S \sim x$ for some ordinal $x$.

Let $\card S$ denote the cardinality of $S$.

Then:


 * $\card S \in \On$

where $\On$ denotes the class of all ordinals.

Proof
If $S \sim x$, then $\set {x \in \On: S \sim x}$ is a non-empty set of ordinals.

It follows that this set has a minimal element, its intersection.

This minimal element is the cardinal number of $S$, by the definition of cardinal number.

Thus, it is an ordinal.