Free Matroid is Matroid

Theorem
Let $S$ be a finite set.

Let $\struct{S, \powerset S}$ be the free matroid of $S$.

Then $\struct{S, \powerset S}$ is a matroid.

Proof
Let $S$ have cardinality $n$.

Let $\struct{S, \mathscr I_{n,n}}$ be the uniform matroid of rank $n$.

From Cardinality of Proper Subset of Finite Set, every subset of $S$ has cardinality less than or equal to $n$.

It follows that $\mathscr I_{n,n} = \powerset S$.

From Leigh.Samphier/Sandbox/Uniform Matroid is Matroid, then $\struct{S, \powerset S}$ Is a matroid.