Set Finite iff Injection to Initial Segment of Natural Numbers

Theorem
Let $S$ be a set.

Then $S$ is finite for some $n \in \N$ there exists an injection $f: S \to \N_{< n}$.

Here, $\N_{< n}$ denotes an initial segment of $\N$.

Necessary Condition
Suppose that $S$ is finite.

By definition, this means there exists a bijection $f: S \to \N_{< n}$.

Then $f$ is a fortiori also the sought injection.

Sufficient Condition
Let $f: S \to \N_{< n}$ be an injection.

Then by Injection to Image is Bijection, $S$ is equivalent to a subset of $\N_{<n}$.

By Subset of Finite Set is Finite, it follows that $S$ is finite.