 As mentioned previously, there are no individual or predicate constants in the formal system of PM, only variables. The Introduction, however, makes use of the example “\(a\) standing in the relation \(R\) to \(b\)” in a discussion of atomic facts (PM, 43). Although “\(R\)” is later used as a variable that ranges over relations in extension, and “\(a,b,c,\ldots\)” are individual variables, let us temporarily add them to the system as predicate and individual constants, respectively, in order to discuss the use of variables in PM.