Can we apply a fictionalist strategy to the metaphysics of property-boundaries? The following thoughts are still very sketchy, and I’d be glad to be put right if I’ve missed something obvious. The aim is to clarify somewhat the motivation for supervaluationism.
Consider first modal fictionalism, which is to a good approximation the doctrine that ‘possibly p’ is true iff ‘according to the modal realist fiction of concrete possible worlds, p is true at some world.’ The fictionalist claims that the possible-world ontology is firmly embedded in our thinking about modality, but nevertheless functions as a useful fiction; fundamentally speaking, there are no possible worlds of the sort that that the modal realist acknowledges, but their existence is a presupposition of our practice of modalizing.
The reason that modal fictionalism is unconvincing is that it fails to explain why modal thinking is so useful, despite involving a presupposition which is literally false. Compare the no-miracles argument in philosophy of science – it would be a miracle if modal thinking worked as well as it does, despite having a false existential presupposition about concrete worlds, just as it would be a miracle if quark science were as successful as it is, despite having a false existential presupposition about quarks.
Now consider an application of this strategy to vagueness. The view I am thinking of is that, to a good approximation, ‘x is F’ is true iff, according to the fiction of precise unknowable boundaries, x falls within the extension of F.’ The semantic fictionalist claims that existence of precise unknowable boundaries is firmly embedded in our thinking about vagueness, but nevertheless functions as a useful fiction; fundamentally speaking, there are no precise boundaries of the sort that the epistemicist acknowledges, but their existence is a presupposition of our practice of modalizing. The commitment to the precise and autonomous extension-fixing mechanism envisaged by epistemicism is merely a fictional one, and semantic theory ought to be treated purely instrumentally.
The thought is that the demand for explanation here is easier to resist than the demand for explanation in the case of modal fictionalism. We want to be able to use the simplest possible logic for our language – taking for granted the existence of sharp boundaries allows us to use classical logic in full generality. This could be taken as an independent explanation of why predicative language is a much more powerful tool if we make the presupposition of sharp boundaries. And the explanation which is analogous to the scientific realist’s explanation for the success of science – that is, that there is indeed a landscape of semantic facts out there determining sharp boundaries – seems much less plausible in the current case. So the objection which in my opinion cripples modal fictionalism leaves semantic fictionalism untouched.
However, another objection looms large. Which fiction should we use? There are myriad precise-boundary fictions available, which draw the lines slightly differently from one another. It seems that a small difference in the details of the fiction wouldn’t make a significant difference in the practical utility of the fiction – classical logic holds whichever fiction we choose, and the differences under consideration can be made small enough so as to not clash significantly with use. The objection is that any choice of fiction would be arbitrary.
This arbitrariness, I think, is fatal to semantic fictionalism. It doesn’t get such a good grip against modal fictionalism, because arguably the Lewisian modal realist ontology is to at least a significant degree non-arbitrary. But the arbitrariness of the sharp boundaries, which is so intuitively problematic for epistemicism, recurs for semantic fictionalism as the arbitrariness of which is the ‘correct’ fiction.
The thought that any particular placing of the sharp boundaries is somewhat arbitrary is the main motivation for supervaluationist approaches to vagueness. According to supervaluationism, it is indeterminate which of some set of precise fictions is the correct one – there is simply no fact of the matter. Nonetheless, when evaluating logical reasoning, logically valid arguments will be valid whichever fiction we choose – which is what matters for the theoretical utility of language-use.
How supervaluationism should be developed is a further question, and a difficult one. My preference is for views which reject the truth-supertruth identification – I’ll try to say more about this in future posts.