Provable Consequence of Theorems is Theorem

Theorem
Let $\PP$ be a proof system for a formal language $\LL$.

Let $\FF$ be a collection of theorems of $\PP$.

Denote with $\map {\mathscr P} \FF$ the proof system obtained from $\mathscr P$ by adding all the WFFs from $\FF$ as axioms.

Let $\phi$ be a provable consequence of $\FF$:


 * $\vdash_{\mathscr P} \FF$
 * $\FF \vdash_{\mathscr P} \phi$

Then $\phi$ is also a theorem of $\mathscr P$:


 * $\vdash_{\mathscr P} \phi$

Proof
We have that $\phi$ is a provable consequence of $\FF$.

Hence it is a theorem of $\map {\mathscr P} \FF$, the proof system obtained from $\mathscr P$ by adding all of $\FF$ as axioms.

Now in the formal proof of $\phi$ in $\map {\mathscr P} \FF$, both axioms and rules of inference are used.

Each rule of inference of $\map {\mathscr P} \FF$ is also a rule of inference of $\mathscr P$.

Similarly, by construction, each axiom of $\map {\mathscr P} \FF$ is either an axiom of $\mathscr P$ or an element of $\FF$.

But the elements of $\FF$ are theorems of $\mathscr P$, each of which thus has a formal proof in $\mathscr P$.

By definition, the rules of inference of a proof system do not distinguish between theorems and axioms.

Therefore the formal proofs of the following can be combined:


 * $\vdash_{\mathscr P} \FF$
 * $\vdash_{\map {\mathscr P} \FF} \phi$

by letting the latter follow the former.

This yields a new formal proof, which is entirely formulated in $\mathscr P$.

This is the desired formal proof of $\phi$ from $\mathscr P$, and we conclude:


 * $\vdash_{\mathscr P} \phi$