Thursday, September 01, 2005

Whole Lotta Worlds

I enjoyed reading David Lewis’ On the Plurality of Worlds. Of course, for me it was like entering a theater in the third act: my eclectic self-education has big holes in it which inhibit my comprehension of such a work.

The usefulness of expressing modal concepts in terms of possible worlds seems clear enough (here’s a blog post which outlines this well – hat tip to the latest philosophy carnival). David Lewis argues that the value and utility of possible worlds in modal thinking points to a metaphysical or ontological argument that the full set of possible worlds exists concretely. In the book he defends this thesis.

Most other philosophers would say possible worlds are abstract, like mathematical objects. Even setting aside that the ontological status of mathematical objects is highly controversial topic, Lewis moves from his usual modest tone toward vehemence in criticizing these views as insufficient in supplying the full structure needed for possible worlds; further the question of how the actual world is selected from the set of abstract possibilities is either inadequate or “magical” in competing accounts.

In other words a sufficient proposal for a modal metaphysics either needs to have Lewis’ ontological panoply of concrete worlds, or else we need a fuller account of the ontological status of the abstract world along with a detailed mechanism for selecting the actual world (in Lewis’ scheme, “actual” just means the world we happen to be in, but there is nothing else special about it relative to the other worlds). Note that this means a system with only one actual concrete world and no ‘abstract realm+selection’ endowment is simply an inadequate metaphysics. While Lewis takes this for granted, I think it's interesting that seemingly only a small subset of philosophers would endorse either the Lewisian system or an abstract alternative with full mechanisms for addressing the Lewisian critique. They are presumably content to deal with modality in epistemological and linguistic arenas unburdened by the metaphysical worries.

While Lewis doesn’t explicitly address it much in this book, the other way to parse the metaphysics is in terms of causality. Lewis’ approach enables him to be a Humean about causality. Everything in the world just happens. If we want to talk about what counterfactual situations could have possibly happened to us, say, then we are talking about things that happen to our counterparts in other worlds. If we want to say there is only one concrete world where things happen and these are caused in a way which could have happened differently (they are contingent things), then we need a model of “real” causation that can handle this selection process. These two alternatives exclude the common assumption many seem to share that there is only one world, but a simple billiard ball notion of causation is all we need. In such a world there is no possibility or contingency at all. Again, this would be an inadequate metaphysics.

So, what do I think of Lewis’ proposal? I don’t like the Humean/fatalistic aspect of it. I would like to think a selection process happens rather than think that every possible world exists in an even-handed way, and we just happen to be in one. But at this point this is a preference on my part, not an argument.

What I get out most out of this is what I’ve been emphasizing in this post: Lewis is an obstacle to those who think they can “get away with” a minimalist metaphysics like single-world physicalism. It’s not enough.


Richard Jordan said...

Thanks for the pointer to this. I need to give it a read.

Personally I lean towards existence of all possible worlds rather than selection, but an adequate description of what constitutes a possible world becomes an interesting point of discussion then... and I think this is an area where mathematicians and philosophers need to get back to working more closely together...

I also tend to merge this with my inclination towards a link between consciousness and causality. This ties together the two issues by the simple expedient: a world which lacks causality will be unable to produce conscious observers and therefore its existence cannot be experienced.

Finally - I think that there is a blur of consciousness not a clean line one side of which is conscious and the other side of which is not... it surprises me how so many philosophers in so many fields seem to fall into such binary thinking. However it does pose questions of what partially conscious means.

Steve said...

"a world which lacks causality will be unable to produce conscious observers..." This is a really good insight I think. If the whole spacetime continuum start to finish just is, it seems to follow that conscious experience has no role to play.

Peter said...

Hard to imagine that a world without causality could be coherent at all. My existence at this moment is caused by my existence at the preceding moment (isn't it?); without that kind of basic consistency, it seems the world would collape into noise, or nothing.

I've never felt altogether happy with reasoning about possible worlds. When someone like Putnam postulates a world in which water it XYZ, not H2O, my instincts revolt - 'water can't be anything other than H2O!' If you start with a premise you know to be false, where do logic and intuition go? But I know that dealing with counterfactuals within a single world is really just as bad. The most tractable cases, I think, are those where possibiltiy can be analysed as compatibility with a given set of laws - the laws of mathematics, or physics. But then people want to invoke metaphysical possibility, and I have no idea what the laws of metaphysics might be.

But I think I'd better shut up now before I lapse into incoherence myself.

Steve said...

I'm still surprised by how problematic causality is. Our common sense perception of it is found nowhere in physics. And philosophers mainly have not seemed to be able to overcome Hume. You need some extra (some would say extravagant) metaphysical structure to handle it (I owe much of this insight to Gregg Rosenberg).