Section 02: Derived rules
The rules of the natural deduction system are meant to be systematic. There is an introduction and an elimination rule for each logical operator, but why these basic rules rather than some others? Many natural deduction systems have a disjunction elimination rule that works like this:
Let’s call this rule Dilemma (DIL) It might seem as if there will be some proofs that we cannot do with our proof system, because we do not have this as a basic rule. Yet this is not the case. Any proof that you can do using the Dilemma rule can be done with basic rules of our natural deduction system. Consider this proof:
\(\mathcal{A}\), \(\mathcal{B}\), and \(\mathcal{C}\) are meta-variables. They are not symbols of SL, but stand-ins for arbitrary sentences of SL. So this is not, strictly speaking, a proof in SL. It is more like a recipe. It provides a pattern that can prove anything that the Dilemma rule can prove, using only the basic rules of SL. This means that the Dilemma rule is not really necessary. Adding it to the list of basic rules would not allow us to derive anything that we could not derive without it.
Nevertheless, the Dilemma rule would be convenient. It would allow us to do in one line what requires eleven lines and several nested subproofs with the basic rules. So we will add it to the proof system as a derived rule.
A derived rule is a rule of proof that does not make any new proofs possible. Anything that can be proven with a derived rule can be proven without it. You can think of a short proof using a derived rule as shorthand for a longer proof that uses only the basic rules. Anytime you use the Dilemma rule, you could always take ten extra lines and prove the same thing without it.
For the sake of convenience, we will add several other derived rules. One is modus tollens (MT).
We leave the proof of this rule as an exercise. Note that if we had already proven the MT rule, then the proof of the DIL rule could have been done in only five lines.
We also add hypothetical syllogism (HS) as a derived rule. We have already given a proof of it on p. 109.