Cardinality of Set is Finite iff Set is Finite

Theorem
Let $A$ be a set.

$\left\vert{A}\right\vert$ is finite

$A$ is finite

where $\left\vert{A}\right\vert$ denotes the cardinality of $A$.

Proof
Definition of cardinal:
 * $(1): \quad \left\vert A \right\vert \sim A$.


 * $\left\vert{A}\right\vert$ is finite


 * $\exists n \in \N: \left\vert A \right\vert \sim \N_n$ by definition of finite set


 * $\exists n \in \N: A \sim \N_n$ by $(1)$ and Set Equivalence is Equivalence Relation


 * $A$ is finite by definition of finite set.