  If we formalise a theory in a first-order language: ∏(O1,…,On;T1,…,Tm), where the Os are the observational terms and the Ts are the theoretical terms, then the corresponding Ramsey sentence is ∃t1,…,tm∏(O1,…,On;t1,…,tm). Thus the Ramsey sentence only asserts that there are some objects, properties and relations that have certain logical features, satisfying certain implicit definitions. It is a higher-order description, but ultimately connects the theoretical content of the theory with observable behaviour. However, it is a mistake to think that the Ramsey sentence allows us to eliminate theoretical entities, for it still states that these exist. It is just that they are referred to not directly, by means of theoretical terms, but by description, that is via variables, connectives, quantifiers and predicate terms whose direct referents are (allegedly) known by acquaintance. Thus Maxwell (and Russell) claimed that knowledge of the unobservable realm is limited to knowledge of its structural rather than intrinsic properties, or, as is sometimes said, limited to knowledge of its higher-order properties. It is arguable that this is the purest structuralism possible, for the notion of structure employed refers to the higher-order properties of a theory, those that are only expressible in purely formal terms.