Finite iff Cardinality Less than Aleph Zero

Theorem
Let $X$ be a set.

Then $X$ is finite $\left\vert{X}\right\vert < \aleph_0$

where:
 * $\left\vert{X}\right\vert$ denotes the cardinality of $X$
 * $\aleph_0 = \left\vert{\N}\right\vert$ by Aleph Zero equals Cardinality of Naturals.