ABSTRACT
In considering various common forms of inference, the authors have examined a few of the schemata; and we have given a fairly informal account of the conditions which a properly put together schema (whether valid or not) must satisfy. It is now time to give a precise and systematic account of these conditions, and thereby to specify the field of Propositional Calculus (PC) in a different way. It will be convenient to adopt a terminology widely used in the literature, and say that any properly put together schema is a well-formed formula. It may help to think of a well-formed formula as a grammatical sentencein the symbolic language of PC – and of the rules we shall set up as its rules of grammar. It is essentially a set of rules for building up successive members of a class by performing certain operations on a stated basis.
