Friday, February 09, 2007

Woolsey's Modal Realism

I recently discovered a paper on modal realism posted by Damon Woolsey, and I liked it very much. Here is a brief take on it.

Woolsey endorses modal realism, but has some problems with David Lewis’ version. He first considers the alternative of branching worlds as a way of reducing the extravagant number of non-overlapping causally distinct worlds in Lewis’ theory. This gives one a system of world-lines in a tree-like structure. A virtue of this may be that it is consistent with our intuition of indeterminism and an open future (for Lewis, the actual and possible worlds are deterministically fixed). On the other hand, the idea of a persistent individual persisting across different world-lines is problematic (vs. Lewis’ use of counterpart theory). Also, with branching worlds, we must accept that the “worlds” are constructions of a sort, rather than really unified worlds. Given this, we should bite the bullet and give upon fundamental possibilia as unified worlds and consider them instead as the set of possibilities considered from one’s particular point of view.

Without postulating all the details of a causal theory or a theory of mind, Woolsey asks us to consider that each of us comprises a causal nexus. We have direct acquaintance with the state of affairs which enter into our nexus. There could be various possibilities going on at points distant from us (patterns of sand in the Gobi Desert) which are consistent with our local facts. So, real possibilities are defined as those states of affairs which can mutually co-exist with a given causal nexus. Given this vision, a possible world is derived as a specific “maximally consistent” state of affairs. The paper has a good discussion fleshing out these notions.

To digress for a moment: one thing I like about this idea is how it coheres with the relational interpretation of quantum mechanics. Possibilia are real, but this reality is relative to a given quantum system=causal nexus (as opposed to a modal realism which features truly branching worlds, which would be consistent with the many worlds interpretation).

In Woolsey’s view, one can derive worlds also by tracing the chain of events back and looking at what other possible world-lines could have happened (he calls this the “causally constrained principle of recombination”). I like this, because it coheres with my discussion in prior posts about how we can know about metaphysical possibilities (see this post, including the comment thread, also this one on Alexander Pruss’ theory). The idea is that the real possibilia that ground our modal intuitions are those adjacent to us. Possible worlds and the unicorns and such which inhabit them are more distant entities which are extrapolated from this raw material. Our conceivability apparatus “rewinds the tape” from our current situation to infer the nature of more distant possibilities. It gets more difficult to imagine how this works when we go beyond nomologically possible worlds and individuals, but I trust our intuition that physical laws are contingent as well, and that our ability to conceive the full metaphysically possible landscape is basically sound. After all, on this account, the intuition is a form of knowledge grounded by contact with real adjacent possibilia. (One area where Woolsey differs is that he is reluctant to endorse possibilia which go beyond the ones consistent with our physical law and the initial state of our world).


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Anonymous said...

I'd like to offer two simple arguments against the theory that the worlds of modal realism could be the worlds of quantum MWI.

1) The worlds of quantum MWI are all quantum-possible worlds. The worlds of modal realism are all logically possible worlds. Ergo, the worlds of quantum MWI are not the worlds of modal realism.

2) Counterlegals (If the laws of nature had been different in such and such a way, then the world would have been different in such and such a way) are often true. Here's a counterlegal: If the laws had been appropriately different, then the worlds of quantum MWI would not have existed. This is obviously true. But if the worlds of quantum MWI are the worlds of modal realism, then this counterlegal entails that possibilities might not have existed. But it is not the case that possibilities might not have existed. Ergo the worlds of quantum MWI are not the worlds of modal realism.

Steve said...

Leaving aside the fact that I think MWI is incorrect anyway, let me look at your arguments.

1. I agree, with one caveat. Physical laws are indeed contingent, and therefore so-called nomological possibility is a subset of logical possibility (which may or may not be the same thing as metaphysical possibility - I think it is, but that's another discussion).

My caveat is to point out that most of the physical laws we're used to talking about emerge at a much higher level than the most basic quantum level. Could elementary quantum mechanical processes in the very early universe have resulted in different emergent particles and forces? I think so. So, I guess my point is that the QM laws might be closer to the level of logical laws than we're used to thinking when talking about other physical laws.

2. I think the point here is similar and is based on the contingency of qm "laws".

To conclude, it's hard for me to disagree with what you say, but I still am interested in the idea that quantum indeterminancy could be "raw material" from which we extrapolate/construct logically possible worlds.

Anonymous said...

Agreed that it's an interesting idea.

The first argument does lose force if quantum-possibility can serve as the outermost sphere of possibility. Whether or not there are intuitively possible (but not quantum possible) worlds would be an interesting direction of investigation.

I think that the second argument is powerful. A proponent of the identification of quantum possible worlds with possibilities simpliciter either has to deny that quantum possible worlds might have failed to exist, either by denying specific counterlegals or by denying counterlegals in general, or he must accept that possibilities might have failed to exist. Either way, he pays a tremendous intuitive cost.

Steve said...

I see your point. I'm tempted to grasp the first horn and say that quantum level possibility could not have failed to exist because it is the outer edge of possibility, but this is a somewhat crazy speculative idea.

By the way, thank you for commenting here.
- Steve