 Operators, however, are to be distinguished from functions in at least that aspect: genuine iteration of operators—the sentential operators of propositional logic are a prime example—is possible without supposing a change of sense or reference from one token to the other. What is their meaning or referent then? Wittgenstein denies they have any referents, this is a generalisation of his claim that the logical constants are not representatives. Peter Hylton (1997: 96–98) argues that Wittgenstein in the Tractatus has Russellian propositional functions in mind when he talks of ‘functions’ and is at pains to distinguish operators from these “substantial” entities. Russellian propositional functions are not the same as ordinary mathematical functions, the model for Frege’s notion of function. Rather they are structured entities, structurally related to the propositions which are their values— gappy states of affairs might be one way to think of them. Operators, by contrast, do not stand for any such entity, they are not parts nor any sort of ingredients of propositions, they “leave no trace” in them. 