I’m teaching a seminar on David Lewis in the spring, using “Daniel Nolan’s book”:http://www.amazon.com/exec/obidos/ASIN/0773529314/ref=nosim/caoineorg-20 as one of the texts, but primarily using primary sources. (Note the link is now to Amazon US, not to Britain.) There’s one primary historical claim I want to make through the course, and I might post stuff here as I write it to try and back up this claim.

The claim is that concrete possible worlds aren’t that important to Lewis’s overall philosophy.

Now this might seem like a rather odd claim to make, since the existence of concrete possible worlds is the core claim of Lewis’s longest and most important book, and most summaries of Lewis start with this belief as the lead. So I’m swimming a little against the interpretative tide, such as it is at this early stage. But I do have something like an argument for my preferred interpretation, or perhaps better, preferred sense of what is central and what is relatively peripheral.

Although Lewis held fairly consistently to most of his views throughout his career, he was amenable to changing his mind in the face of good argument. He was constantly tinkering with the analysis of causation, he accepted some tinkering to the analysis of intrinsicness, and most importantly, he abandoned egalitarianism about properties. So we can coherently, even easily, imagine him changing his mind on some of his conclusions.

Now for any paper or work of Lewis’s, do the following thought experiment. Imagine someone giving him a convincing argument that the core conclusion of the paper failed, shortly after the paper appeared in print. Try to follow through then how many changes would have been needed to later work if he’d accepted this criticism, and hence not been able to presuppose the truth of his earlier claims.

In some cases the changes would be immense. It is almost impossible to imagine what Lewis’s work would look like if he didn’t have the Ramsey-sentence definition of theoretical terms, and his convention-based account of language to work with throughout. Similarly we would need massive revisions if he’d been convinced that Humean supervience was fundamentally flawed, or that egalitarianism about properties was true after all, or that one of the paradoxes from chapter 2 of _Plurality_ was fatal to the possible worlds apparatus. These I think are the five core claims that Lewis’s philosophy is built around, and if he’d given any of them up, it would have forced massive revision to the rest of his work.

Not so for concrete possible worlds. If someone had written a version of ersatzism in 1987 that Lewis thought gave us all we needed from a theory of possible worlds, and wasn’t vulnerable to the chapter 3 objections, then he could have accepted it, and, more importantly, he would have had to make next to no substantive changes to anything else he wrote. What is of broad philosophical importance for Lewis (and others such as Stalnaker and Jackson are in this camp too) is that the possible worlds framework is coherent and applications of it can be philosophically enlightening. Lewis had opinions on the metaphysical foundations of that framework, but they weren’t central to the philosophical picture he gave us.

The test I’m using for importance isn’t perfect, because it isn’t clear always how to apply it. I’ve been deliberately silent in what I’ve said above about the importance of the analysis of counterfactuals and of causation to the picture, as it is being sketched here. I tend to think that in both cases the details (in some sense of detail) are not important, any more than the details of the account of intrinsicness or colour or dispositions or chance or laws are important. (Which is not to say they don’t at all matter – Lewis worked hard on finding reasons to support particular analyses of all of these.) But both causation and counterfactuals seem more central to the project than these other cases, especially counterfactuals. Saying just why I think that needs more work, and hopefully I’ll be able to have something more coherent by the time next spring rolls around.

I have a vested interest in this interpretation being taken seriously, because broadly speaking I’m a “Canberra Credo”:http://www.st-andrews.ac.uk/~dpn/docs/credo.htm guy, and “We [Credoers] believe in the substantial correctness of the doctrines of David Lewis about most things (except the nature of possible worlds).” It would be nice if (a) we could argue that this is a coherent position, which it isn’t if concrete possible worlds are central to Lewis’s picture, and (b) one which involves only a small deviation from Lewis’s theories, since it’s odd to look up to someone on so many matters while disagreeing with them about what is fundamental to their work.