Quantifier-Free Formula of Arithmetic is Provable/Corollary

Theorem
Let $\phi$ be a WFF in the language of arithmetic.

Suppose $\phi$ contains no quantifiers.

Then exactly one of:
 * $\phi$
 * $\neg \phi$

is a theorem of minimal arithmetic.

Proof
By Law of Excluded Middle, either $\N \models \phi$, or $\N \models \neg \phi$.

By Quantifier-Free Formula of Arithmetic is Provable, whichever one holds is a theorem of minimal arithmetic.

Now, suppose that $\N \models \phi$.

Then, $\N \not\models \phi$ by definition of value of formula.

Thus, by the definition of sound proof system, $\neg \phi$ is not a theorem.

Therefore, $\phi$ and $\neg \phi$ cannot both be theorems.