Consider two arguments in SL:
\(P\) ∨ \(Q\)
\(P\) → \(Q\)
Clearly, these are valid arguments. You can conﬁrm that they are valid by constructing four-line truth tables. Argument A makes use of an inference form that is always valid: Given a disjunction and the negation of one of the disjuncts, the other disjunct follows as a valid consequence. This rule is called disjunctive syllogism.
Argument B makes use of a diﬀerent valid form: Given a conditional and its antecedent, the consequent follows as a valid consequence. This is called modus ponens.
When we construct truth tables, we do not need to give names to diﬀerent inference forms. There is no reason to distinguish modus ponens from a disjunctive syllogism. For this same reason, however, the method of truth tables does not clearly show why an argument is valid. If you were to do a 1024-line truth table for an argument that contains ten sentence letters, then you could check to see if there were any lines on which the premises were all true and the conclusion were false. If you did not see such a line and provided you made no mistakes in constructing the table, then you would know that the argument was valid. Yet you would not be able to say anything further about why this particular argument was a valid argument form.
The aim of a proof system is to show that particular arguments are valid in a way that allows us to understand the reasoning involved in the argument. We begin with basic argument forms, like disjunctive syllogism and modus ponens. These forms can then be combined to make more complicated arguments, like this one:
(1) ¬\(L\) → (\(J\) ∨ \(L\))
By modus ponens, (1) and (2) entail \(J\)∨\(L\). This is an intermediate conclusion. It follows logically from the premises, but it is not the conclusion we want. Now \(J\) ∨ \(L\) and (2) entail \(J\), by disjunctive syllogism. We do not need a new rule for this argument. The proof of the argument shows that it is really just a combination of rules we have already introduced.
Formally, a proof is a sequence of sentences. The ﬁrst sentences of the sequence are assumptions; these are the premises of the argument. Every sentence later in the sequence follows from earlier sentences by one of the rules of proof. The ﬁnal sentence of the sequence is the conclusion of the argument.
This chapter begins with a proof system for SL, which is then extended to cover QL and QL plus identity.