 where \(\delta\) is a one-place propositional functor, out of the same syntactic stable as negation or necessity. This thesis is an expression of the law of extensionality for propositional expressions. If \(p\) and \(q\) are replaced by complex expressions \(x\) and \(y\) the thesis can be used to enable definitions to be given in the implicational form \(C\delta x\delta y\).