Isomorphic Ordinals are Equal/Proof 1

Proof
Let $S \cong T$.

that $S \ne T$.

Then from the corollary to Relation between Two Ordinals, either:
 * $S$ is an initial segment of $T$

or:
 * $T$ is an initial segment of $S$.

But as $S \cong T$, from Well-Ordered Class is not Isomorphic to Initial Segment, neither $S$ nor $T$ can be an initial segment of the other.

From this contradiction it follows that $S = T$.