I find mereological nihilism an attractive view. All there are are simples: there’s no mereological complexity to the world. I feel no need to say as a result of this that there are no tables or chairs, provided we don’t take those claims to be perspicuously describing fundamental reality. ‘There are tables’ might be a true sentence of English; but it is being made true not by a mereologically complex object, but simply by a collection of simples arranged a certain way. (See my paper and Robbie’s paper on fundamentality.)
I’m also attracted to the view that there aren’t really any structural universals. All there are are the perfectly natural basic universals. That’s not to say that there’s no methane; it’s just to say that claims about methane will be made true not by a structured universal METHANE but, ultimately, by the pattern of instantiation of the basic (let us suppose) universals HYDROGEN and CARBON.
One type of argument you hear against views like this is that we have to believe in the complex things because there might not be the entities at the bottom level. So we have, e.g., Sider and Armstrong, arguing that we’ve got to believe in mereologically complex entities because there might be no simples, i.e. the world might be gunky. And we have, e.g., Lewis and Armstrong arguing that we’ve got to believe in structural universals because there might be no basic ones. (I’m using a bit of poetic license here: Lewis didn’t really believe in structural universals – but he thought this was the best argument to believe in them.)
There are two ways to read the complaint. One is to read the ‘might’s in the above as meaning metaphysical possibility, one is to read them as meaning epistemic possibility.
I find the former form of the argument unconvincing. For starters, the metaphysical possibility of gunky worlds, or worlds with infinitely descending chains of structural universals, is far from a datum. (See this paper and this paper by Robbie, which attempt to explain away the illusions of possibility in each case.) But also, even if these are genuine possibilities, I only see a reason to believe that there might have been mereological complexity and structural universals; I don’t see any reason to think that the world actually contains either kind of complex entity. The two positions I confessed my attraction to are claims about how the world actually is, not how it must have been; the possibility of infinite complexity doesn’t give me any reason to accept the actuality of infinite complexity. (See my paper on the contingency of composition.)
What if the ‘might’s are read as epistemic modality? There the complaint is that we have no right to reject the existence of mereologically complex objects or structured universals because we have no guarantee that there are in fact the mereological simples, or basic universals, that there would need to be.
This is, I think, how Armstrong intends the objection (at least sometimes). As he sees it, I think, we’ve got no right just to assume that there are simples or basic universals. That would be a priori ontology, and therefore suspicious! We shouldn’t build theories on the assumption that there are the entities at the bottom level, then, and this means we have to allow that there are the complex entities.
I’ve heard something like that argument from quite a few people, but I don’t find it at all moving. Yes, there might not be any simples or basic universals. My theory might be wrong! There’s no a priori guarantee that there are simples or basic universals. So what? There’s no a priori guarantee that there are complex objects or structural universals either, so where’s the asymmetry? In accepting the two theories above I close off the epistemic possibility that there are no simples or basic universals, but in accepting Armstrong’s theory I close off the epistemic possibility of there being no complex objects or structural universals: why is one better than the other? Every theory closes off epistemic possibilities, unless it is a theory that tells us nothing about the world. So why is it a good objection to the above theories that their truth requires the existence of entities that we have no guarantee exist? Sure, I have no guarantee that there are simples or basic universals. It’s a hypothesis that there are; that hypothesis will then be judged just like any other: on the balance of costs and benefits.
Why might you think there was an asymmetry between reliance on the existence of the simple ontology and reliance on the existence of the complex ontology? You might think that there is an a priori guarantee of the existence of the complex ontology but not the simple ontology? Why? Well in the case of mereology, the existence of the complex objects is guaranteed by the axioms of classical mereology but the existence of simples is not. But that’s not convincing. The question then is simply: why believe in the axioms of classical mereology? They close off epistemic possibilities as well. To claim that they’re a priori looks no better to me than the claim that it’s a priori that there are simples. Assume the axioms of classical mereology and construct your theory on that basis by all means; but then I have as much right to do the same with the assumption that there are the simples – and then to the victor the spoils.
Perhaps the asymmetry is meant to be that the hypothesis that there are the complex objects is empirically sensitive in a way the hypothesis that there are the simple objects isn’t. But I can’t see any reason to think that that is true. If anything it’s the other way round: there would be no observable difference in the world were there complex objects as opposed to simples arranged a certain way, but if scientists are unable to split the lepton (or whatever) that gives us some reason to believe that leptons are mereologically simple. (I don’t really believe that, but some people do.)
So where’s the asymmetry? Any suggestions? Is the metaphysician who relies on the existence of simples doing anything worse than the metaphysician who relies on the existence of complexity?
Does the asymmetry enter if we also assume that facts about mereological composition are metaphysically necessary (as Lewis and Armstrong at least do)? (I see that you don't want to do this though.)
If we accept that facts about mereological composition are necessary and if the gunky world is an epistemic possibility then we have reason for not believing any theory of mereological composition that relies on simples.
But if we accept that facts about mereological composition are necessary and a world without complex objects is an epistemic possibility then we need a bit more to get an argument against e.g. the person who believes in universal composition. We need not just the possibility of there being a world with no complex objects but we need the possibility of there being a world with more than one simple and no complex objects.
So there is perhaps a small asymmetry here: a slightly more fleshed out intuition/epistemic possibility is needed to argue against the friend of universal composition.
That said both arguments look rather close to being question begging.
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