Set has Rank

Theorem
Let $S$ be a set.

Then $S$ has a rank.

Proof
Let $G$ be the smallest transitive set containing $S$ as a subset.

By Set Contained in Smallest Transitive Set, $G$ must exist.

By Transitive Set Contained in Von Neumann Hierarchy Level, $G \subseteq V_i$ for some ordinal $i$.

Therefore:
 * $G \in V_{i+1}$

We have that the ordinals are well-ordered.

From Proper Well-Ordering determines Smallest Elements, there exists a smallest ordinal $k$ such that $G \in V_{k+1}$.

Hence, by definition, $G$ has rank $k$.