A Thought Experiment against Humean Supervenience: Conceivability & Intuitions

One thought experiment that objectors against Humean Supervenience are quick to suggest is that of two possible worlds which both contain only one existing material object that always moves in a constant velocity. The thought experiment goes: in the first world, the particle is governed by the law that all particles move with constant velocity and in the second world, the particle is governed by the law that all particles obey Newton's Second Law; in other words, in the second (and not the first) world, if another particle had existed there, they may have had an effect on one another's velocity.

Is this a good counterexample via thought experiment? In a recent paper, Ned Hall tries to construe this counter-example as relying on the 'Conceivability entails Possibility' principle. And he therefore dismisses it. It is certainly true that the set-up of the thought experiment involves the (C-->P) principle, insofar as it must be committed to it being possible that there could be a world with only one particle and a world differing from another in only its modal facts.

But although I whole-heartedly dismiss the (C-->P) principle, I'm not so sure that a counterexample wherein two different objects in two distinct possible worlds are intrinsic duplicates and yet differ in their nomic-profiles is to endorse a possibility only on the basis of conceivability. Why is this such a leap in conceivability? I think the non-reductionist example is meant to (or at least, should aim to) exploit a seperate intuition; not the C-->P intuition. The intuition is: how a particular object (or kind of event) has been, is, or will be says nothing about ways it might have been.

It won't do to simply dismiss the non-reductionist thought experiment on the basis that it assumes that one world could differ in its laws while another one does not. For while this is assumed, and while it is an assumption that tips the scales, so to speak, it is not an altogether unwarranted assumption. And the onus is on the Humean to explicate the ways in which this thought example goes wrong. It won't be enough for the Humean to claim that the two worlds differing in their Laws is just impossible, any more than it will do for the non-reductionist to claim that it is possible. The non-reductionist position exploits an intuition, one that, for all I can tell, is fairly plausible, while the Humean position combats this intuition, but with no reason other than the fact that it does not accord with his theory.

The Status of 'Brute Facts'

Disregarding any theory that posited 'brute facts' - excepting 'identity' - used to be a common thread throughout my philosophical career. As any good Leibnizian, I am deeply committed to some form of the Principle of Sufficient Reason afterall. But as my thinking about metaphysics becomes more and more naturalized, I find myself not only denying this practice but also finding it just plain mistaken metaphysics.

Granted, it would be lovely if we could do away with 'brute facts'. But not only are there philosophical reasons for thinking this an impossible project - think: first mover - but there are good reasons for thinking that primitives ought to be had at a much 'higher level' of ontology. I for one, as a sortal essentialist, think that explanation stops at natural kind identities. So asking questions like "but why is this object this natural kind?" are not pointless, but metaphysically meaningless. You might as well be asking "but why is x identical to x?"

At the recent 'Philosophy of Cosmology' conference at St. Anne's College at the University of Oxford, Paul Davies gave an interesting talk. The gist was: we can always ask 'why' and physics may not always provide an answer. I approached him after his talk and asked him if he would ever be happy with primitives in physics, or philosophy for that matter. He insisted that the questions he raised were valid and important questions for physicists to ask themselves. Now, I agree with that, but I also recognize that there are primitives and then there are primitives. If we have a proposed primitive that is capable of theoretical reduction, then we have no primitive at all. But looking for contrastive reasons for every proposed primitive is, more often than not, a futile effort. In particular, Davies asked, "but why this law, rather than some other?"

I have recently been re-reading sections of Richard Swinburne's 'The Existence of God'. An amazing book to be sure, and there's no doubt that Professor Swinburne has done much to make theological belief intellectually credible. But in his chapter on 'The Nature of Explanation', he insists on these kinds of questions. In discussing the causal powers view of natural laws, he complains that '...the law does not explain why these substances have those powers'. But if these powers are the foundations for the natural laws - there just isn't any worth in asking such questions. If Swinburne is looking for an answer to "why negative charge repels negative charge, rather than attracting it", he is simply assuming that nature has available to it all of the content of the span of logically possible worlds. This is the only thing that can be going on in these types of questions: one must be a Humean about possibility.

If you think there's a meaningful answer to the question "why does negative charge repel negative charge, rather than attract it?", you must think that it is in fact possible that negative charge might have attracted other negative charges. But why think this? As far as I can tell, one would only think this by assuming that logical possibility is the possibility-space for the concrete world. But on a dispositional conception of possibility - to ask such a thing is to be asking a rather ad hoc question. For on this view, the causal powers themselves demarcate what is possible and what is not.

So there is simply no room to ask for any contrastive explanation as to why these powers are as they are. The short answer is: this is a brute fact about reality. And if you think that it is genuinely possible that like negative charges might have attracted, the onus is on you to explain why it is that purely logical possibilities should be countenanced as true de re possibilities for objects in the concrete world. And so you must make the dreaded conceivability entails possibility link.

On 'The Limits of Contingency' & Metaphysical Possibility

An article by Gideon Rosen entitled 'The Limits of Contingency' recently peaked my interest. Rosen makes an interesting distinction between two conceptions of metaphysical possibility - what he calls Standard and Non-Standard. According to the standard conception of metaphysical possibility, a property P is possible for an object x iff P can be possessed by x in a world whose 'form' is the same as the actual world. By form, Rosen means something like the fundamental laws of how objects are composed or how causation goes about - something broadly like 'ontological rules'. According to the non-standard conception of metaphysical possibility, a property P is possible for an object x iff P is logically coherent with the 'what it is to be' of x. That is, only if P does not logically conflict with x's 'individual nature'.

The first thing to notice is that the standard conception requires a certain checking against the conditions of the actual world - a position which I think represents a respectful speculative sobriety. The non-standard version of metaphysical possibility only requires that a property be compatible with the 'nature' of an object and presumably, since an object's nature might very well be intact while it does not exist in the actual world (think Plantingian Essences or Platonic Divine Idea Essences), metaphysical possibility is not constrained by the conditions of the actual world.

I think that the two positions - the standard and non-standard - are, as Rosen claims, very common notions of metaphysical possibility. Those who think that sortals are the de re governors of what is metaphysically possible for an object clearly fall into the non-standard camp, though this seems a rather standard position in modern philosophy. And those who think that metaphysical possibility has something to do with the laws of the actual world will, in most instances, fall into the standard camp; those who do often want to identify metaphysical possibility with physical possibility, broadly construed.

At the risk of being philosophically trite, my sentiments lie with the middle position. In general, I think that the non-standard version is correct - metaphysical possibility must be de re in a significant sense and this sense must have to do with the 'nature' of the object in question. On the other hand, however, the non-standard conception does not appropriately respect the privileged status of actuality - there are no merely possible worlds over which we might peer out and look down upon to inspect the natures of the objects within. Any considerations that we have of this kind - where we think we have real knowledge of the modality of non-existent objects - seem to me to be based solely on our powers of conceptualizing, not on the nature of these objects themselves.

Metaphysical modality, I think, must be about objects and so must also be about their 'definitional natures', but it cannot merely be a matter of logical non-contradiction without consideration of the actual world. On the other hand, it equally cannot concern the 'form' of the actual world, except insofar as 'form' is taken to mean the natures of the objects within it.