 Standard propositional calculus employs neither quantifiers nor variable functors, that is, functors of one or more places taking propositional arguments, but which unlike such constant functors as \(N\) or \(C\) do not have a fixed meaning. Such variable functors act like the predicates of first-order predicate logic except for taking propositional rather than nominal arguments. They thus add to the expressive power of the logic. Leśniewski added both quantifiers and bound propositional and functorial variables to propositional logic, and called the resulting theory protothetic. Leaving prefixed universal quantifiers tacit, it is a thesis of protothetic that