 There is no explicit statement of a rule of substitution in PM. The free variables in the propositional logic of PM may be interpreted as schematic letters, and so the system will require a rule of substitution of formulas. In this article they are to be interpreted as real variables ranging over propositions, in which case instances would be derived by instantiation from generalizations over all propositions. The announcement in the Introduction that propositions are not necessary in what follows and so will be avoided suggests the schematic interpretation of the variables. We follow the variable interpretation in this article, however, in part to allow our notation to follow PM, with p’s and q’s rather than a new vocabulary of schematic letters A, B, etc. This interpretation of the letters as variables will also assist in the presentation of quantificational logic in PM below.