  A position called structural realism, that amounts to an epistemological gloss on traditional scientific realism, was advocated by Grover Maxwell (1962, 1970a, 1970b, 1972). Maxwell wanted to make scientific realism compatible with “concept empiricism” about the meaning of theoretical terms, and he also wanted to explain how we can have epistemic access to unobservable entities. The problem as Maxwell saw it was that theories talk about all sorts of entities and processes with which we are not ‘acquainted’. How, he wondered, can we then know about and refer to them and their properties? The answer that he gave, following Russell, was that we can know about them by description, that is we can know them via their structural properties. In fact, he argues, this is the limit of our knowledge of them, and the meanings of theoretical terms are to be understood purely structurally. The way that Maxwell explicates the idea that the structure of the theory exhausts the cognitive content of its theoretical terms, is to consider the Ramsey sentence of the theory (Ramsey 1929). Ramsey's method allows the elimination of theoretical terms from a theory by replacing them with existentially quantified predicate variables (or names in the case of the influential Lewis 1970). If one replaces the conjunction of assertions of a first-order theory with its Ramsey sentence, the observational consequences of the theory are carried over, but direct reference to unobservables is eliminated.