Minimal Element of an Ordinal

Theorem
The minimal element of any ordinal is the empty set.

Proof
Let $S$ be an ordinal.

Let the minimal element of $S$ be $s_0$.

This exists by dint of an ordinal being a woset.

From Ordering on an Ordinal is Subset Relation, $S$ is well-ordered by $\subseteq$.

So, by definition of an ordinal, $s_0 = \left\{{s \in S: s \subset s_0}\right\}$.

But as $s_0$ is minimal, there are no elements of $S$ which are a subset of it.

So $\left\{{s \in S: s \subset s_0}\right\} = \varnothing$, hence the result.