Even Natural Numbers are Infinite

Theorem
The set of even natural numbers is infinite.

Proof
Let $E$ denote the set of even natural numbers.

$E$ is finite.

Then there exists $n \in \N$ such that $E$ has $n$ elements.

Let $m$ be the greatest element of $E$.

But then $m + 2$ is an even natural number.

But $m + 2 > m$, and $m$ is the greatest element of $E$.

Therefore $m + 2$ is an even natural number that is not an element of $E$.

So $E$ does not contain all the even natural numbers.

From that contradiction it follows by Proof by Contradiction that $E$ is not finite.