 Since a propositional function can be obtained from a proposition ‘by treating one or more of its constituents as variables’, the hierarchy of types and orders is naturally lifted to propositional functions, and it makes sense to speak of order of a function, its order being roughly the order of the value (i.e., a proposition) which the function assumes when it is applied to an argument for which it makes sense. So, for instance, a function which applies to individuals and takes first-order propositions as values is first-order.