Formation rules for PC
In any system of logic it is necessary to specify which sequences of symbols are to count as acceptable formulas—or, as they are usually called, well-formed formulas (wffs). Rules that specify this are called formation rules. From an intuitive point of view, it is desirable that the wffs of PC be just those sequences of PC symbols that, in terms of the interpretation given above, make sense and are unambiguous; and this can be ensured by stipulating that the wffs of PC are to be all those expressions constructed in accordance with the following PC-formation rules, and only these:
- FR1.A variable standing alone is a wff.
- FR2.If α is a wff, so is ∼α.
- FR3.If α and β are wffs, (α · β), (α β), (α ∨ β), (α ⊃ β), and (α ≡ β) are wffs.
In these rules α and β are variables representing arbitrary formulas of PC. They are not themselves symbols of PC but are used in discussing PC. Such variables are known as metalogical variables. It should be noted that the rules, though designed to ensure unambiguous sense for the wffs of PC under the intended interpretation, are themselves stated without any reference to interpretation and in such a way that there is an effective procedure for determining, again without any reference to interpretation, whether any arbitrary string of symbols is a wff or not. (An effective procedure is one that is “mechanical” in nature and can always be relied on to give a definite result in a finite number of steps. The notion of effectiveness plays an important role in formal logic.)
Examples of wffs are: p; ∼q; ∼(p · q)—i.e., “not both p and q”; and [∼p ∨ (q ≡ p)]—i.e., “either not p or else q is equivalent to p.”
For greater ease in writing or reading formulas, the formation rules are often relaxed. The following relaxations are common: (1) Brackets enclosing a complete formula may be omitted. (2) The typographical style of brackets may be varied within a formula to make the pairing of brackets more evident to the eye. (3) Conjunctions and disjunctions may be allowed to have more than two arguments—for example, p · (q ⊃ r) · ∼r may be written instead of [p · (q ⊃ r)] · ∼r. (The conjunction p · q · r is then interpreted to mean that p, q, and r are all true, p ∨ q ∨ r to mean that at least one of p, q, and r is true, and so forth.)