ABSTRACT
The position of Rudolf Carnap in the scientific realism debate has not yet been fully examined and appreciated. The present chapter will aim to do just this. For the Carnap of the 1950s and early 1960s tried hard to show that there was space for an irenic position in the realism debate, a position which could render realist and instrumentalist understandings of scientific theories compatible. In his attempt to develop such a position Carnap took a structuralist turn. He reinvented what came to be called the Ramsey-sentence approach to theories, an approach which was first enunciated by the Cambridge philosopher Frank Plumpton Ramsey in the late 1920s, but had not been fully appreciated until Carnap made it popular. As I show in some detail in this chapter, Carnap thought that the Ramsey-sentence approach can form the basis of a position which takes on board the instrumentalist reluctance to accept that theoretical discourse commits to unobservable entities, while it also accommodates the realist view that theories explain and predict observable phenomena by reference to unobservable entities. On the face of it, the very possibility of such a compromise does not seem to make sense. We shall, however, see that Carnap built a prima facie case for such a compromise. Yet, the resulting position brings together a weak form of realism, according to which only structural claims about unobservable entities can be known and asserted, and an atypical form of instrumentalism, which does not deny that unobservable entities exist. Having explained Carnap’s position in some detail, we shall see that it it open to a damning objection, which was first raised in 1928 by the Cambridge mathematician M.H.A.Newman against Bertrand Russell’s own structuralism. The essence of the objection is that the structuralist claim that only the structure of the unobservable world can be known is either false or else trivial. The message of this chapter is that any meaningful defence of realism presupposes the view that the world is already ‘carved up’ in natural kinds, i.e. that it already possess a natural-kind structure.
