# Deduction Theorem

## Theorem

Let $\mathscr H$ be instance 1 of a Hilbert proof system.

Then the deduction rule:

$\dfrac{U,\mathbf A \vdash \mathbf B}{U \vdash \mathbf A \implies \mathbf B}$

is a derived rule for $\mathscr H$.

## Proof

For any proof of $U, \mathbf A \vdash \mathbf B$, we indicate how to transform it into a proof of $U \vdash \mathbf A \implies \mathbf B$ without using the deduction rule.

This is done by applying the Second Principle of Mathematical Induction to the length $n$ of the proof of $U,\mathbf A \vdash \mathbf B$.

If $n = 1$, then one of the following must occur:

• $\mathbf B \in U$
• $\mathbf B = \mathbf A$
• $\mathbf B$ is an axiom or a theorem of $\mathscr H$

In the first case, obviously $U \vdash \mathbf B$.

By Axiom 1, $U \vdash \mathbf B \implies \left({\mathbf A \implies \mathbf B}\right)$.

By Modus Ponens, $U \vdash \mathbf A \implies \mathbf B$.

In the second case, $U \vdash \mathbf A \implies \mathbf A$ by the Law of Identity.

Finally, in the third case, we have $U \vdash \mathbf B$.

As in the first case, we conclude $U \vdash \mathbf A \implies \mathbf B$.

If $n > 1$, the only other option for arriving at $U, \mathbf A \vdash \mathbf B$ is through Modus Ponens.

That is to say, two earlier lines of the proof contain:

$U, \mathbf A \vdash \mathbf C$
$U, \mathbf A \vdash \mathbf C \implies \mathbf B$

for some WFF $\mathbf C$.

But then these sequents have shorter proofs.

Hence, they satisfy the induction hypothesis.

Thus, we may infer:

$U \vdash \mathbf A \implies \mathbf C$
$U \vdash \mathbf A \implies \left({\mathbf C \implies \mathbf B}\right)$

This allows us to give the following proof of $U \vdash \mathbf A \implies \mathbf B$:

$U \vdash \mathbf A \implies \mathbf B$
Line Pool Formula Rule Depends upon Notes
1 $U \vdash \mathbf A \implies \mathbf C$ Hypothesis
2 $U \vdash \mathbf A \implies \left({\mathbf C \implies \mathbf B}\right)$ Hypothesis
3 $U \vdash \left({\mathbf A \implies \left({\mathbf C \implies \mathbf B}\right)}\right) \implies \left({\left({\mathbf A \implies \mathbf C}\right)\implies \left({\mathbf A \implies \mathbf B}\right)}\right)$ Axiom 2
4 $U \vdash \left({\mathbf A \implies \mathbf C}\right)\implies \left({\mathbf A \implies \mathbf B}\right)$ Modus Ponendo Ponens: $\implies \mathcal E$ 2, 3
5 $U \vdash \mathbf A \implies \mathbf B$ Modus Ponendo Ponens: $\implies \mathcal E$ 1, 4

The result follows by the Second Principle of Mathematical Induction.

$\blacksquare$