Class of All Cardinals is Proper Class

Theorem
The class $\NN$ of all cardinal numbers is a proper class.

Proof
$\NN$ is a small class.

By Cardinal Class is Subclass of Ordinal Class:
 * $\NN \subseteq \On$

Therefore, $\bigcup \NN$ is an ordinal by Union of Subset of Ordinals is Ordinal.

Take $x = \set {y \in \On: y \preccurlyeq \bigcup \NN}$.

By Cardinal Equal to Collection of All Dominated Ordinals, $x$ is a cardinal number and there is no injection $f: x \to \bigcup \NN$.

But since $x$ is a cardinal number, $x \in \NN$, so $x \subseteq \bigcup \NN$ by Set is Subset of Union/General Result.

Therefore, the identity map $I_x: x \to \bigcup \NN$ is an injection.

This is a contradiction.

Therefore by Proof by Contradiction $\NN$ is not a small class.