Richard Marshall interviews Richard Healey in 3:AM Magazine:
3:AM: So does your pragmatism at work in these two cases mean that we should think of quantum mechanics as a realist or an instrumentalist theory or is it a middle way?
RH: Too often contemporary philosophers apply the terms ‘realism’ and ‘instrumentalism’ loosely in evaluating a position, as in the presumptive insult “Oh, that’s just instrumentalism!” Each term may be understood in many ways, and applied to many different kinds of things (theories, entities, structures, interpretations, languages, ….). I once characterized my pragmatist view of quantum mechanics as presenting a middle way between realism and instrumentalism. But by adopting one rather than another use of the terms ‘realism’ and ‘instrumentalism’ one can pigeon hole my view under either label.
In this pragmatist view, quantum probabilities do not apply only to results of measurements. This distinguishes the view from any Copenhagen-style instrumentalism according to which the Born rule assigns probabilities only to possible outcomes of measurements, and so has nothing to say about unmeasured systems. An agent may use quantum mechanics to adjust her credences concerning what happened to the nucleus of an atom long ago on an uninhabited planet orbiting a star in a galaxy far away, provided only that she takes this to have happened in circumstances when that nucleus’s quantum state suffered suitable environmental decoherence.
According to one standard usage, instrumentalism in the philosophy of science is the view that a theory is merely a tool for systematizing and predicting our observations. For the instrumentalist, nothing a theory supposedly says about unobservable structures lying behind but responsible for our observations should be considered significant. Moreover, instrumentalists characteristically explain this alleged lack of significance in semantic or epistemic terms: claims about unobservables are meaningless, reducible to statements about observables, eliminable from a theory without loss of content, false, or (at best) epistemically optional even for one who accepts the theory. My pragmatist view makes no use of any distinction between observable and unobservable structures, so to call it instrumentalist conflicts with this standard usage.
In this view, quantum mechanics does not posit novel, unobservable structures corresponding to quantum states, observables, and quantum probabilities; these are not physical structures at all. Nevertheless, claims about them in quantum mechanics are often perfectly significant, and many are true. This pragmatist view does not seek to undercut the semantic or epistemic status of such claims, but to enrich our understanding of their non-representational function within the theory and to show how they acquire the content they have.