A problem for modal fictionalism?
I am not an expert on modal fictionalism, so probably something is obviously wrong with the following objection. But anyway, here it is.
Modal fictionalism claims that any statement S about possible worlds (and other possibilia) is to be analysed as "According to the possible-world-story, S". Now possible worlds are used in reductive analyses of all kinds of concepts: modality, counterfactuals, causation, laws, properties, propositions, meanings, probabilities, supervenience, fictions, etc. For instance, an analysis of indexicals usually talks about extensions in possible contexts of utterance. If fictionalism is right, then this analysis must in turn be analysed in terms of extension in possible contexts according to the possible-worlds-story. And this seems rather odd. Suppose I propose some theory T of indexicals (or laws or whatever). If fictionalism is right then T is correct iff it is implied by some story about possible worlds. Firstly, intuitively this is not at all what I would have thought my theory was about. Secondly, which possible-world-story is relevant here? If we take the five or six claims about recombination and other worlds being of the same kind as ours usually presented by fictionalists (e.g. Rosen 1990), all the analytic projects mentioned above appear to be doomed: That simple story will not imply anything at all about indexicals, or laws, or causation. Unless of course we extend it by some analysis of these notions. Which analysis? The obvious candidate is the analysis we believe to be true, that is, T. But then all the analytic projects mentioned above come out as trivially true: Even the craziest theory will be good enough to imply itself.
There is another way of looking at the problem: When I propose an analysis of something in terms of possible worlds, I don't seem to be making a claim about my own, private fiction of possible worlds. Take counterfactuals. It doesn't seem plausible that by suggesting a possible worlds analysis, I claim that the truth conditions of every counterfactual ever uttered are determined by what I happen to think about nonexistent possible worlds. But even less am I making a statement about some other philosopher's fiction. I don't claim that the truth conditions of counterfactuals are determined by, say, what Lewis says about possible worlds.
The difference between linguistic ersatzism, where possible worlds are replaced by sets of sentences, and modal fictionalism, where the pluriverse of all worlds is replaced by a large set of sentences describing all worlds at once, appears to be small.