  A third option, alternative to both type-(1) and type-(2) theories, would be an “operationalist” account of the sort advocated by Adams (1984, 1996), where the abstractive process by which boundary elements are derived from concrete observables is explained in terms of “operational tests”. Arguably, however, such an account is best regarded as a parallel story, one that offers an explanation of empirical knowledge concerning boundaries while remaining ultimately neutral with regard to their ontological status.