 Although bound variables ranging over propositions hardly occur in Principia Mathematica (with a prominent exception in *14.3), it would seem that the whole theory of types is a theory of propositional functions. Yet following on the claim that propositions are “not single entities at all”, Russell says the same for propositional functions. In the Introduction to Mathematical Philosophy, Russell says that propositional functions are really “nothing”, but “nonetheless important for that” (Russell 1919, 96). This comment makes best sense if we think of propositional functions as somehow constructed by abstracting them from their values, which are propositions. The propositional function “\(x\) is human” is abstracted from its values “Socrates is human”, “Plato is human”, etc. Viewing propositional functions as constructions from propositions, that are in turn constructions by the multiple relation theory, helps to make sense of certain features of the theory of types of propositional functions in Principia Mathematica. We can understand how propositional functions seem to depend on their values, namely propositions, and how propositions in turn can themselves be logical constructions. The relation of this dependence to the theory of types is explained in the Introduction to Principia Mathematica in terms of the notion of “presupposing”: