Readings on Quidditism
My attempts to get a copy of 'Ramseyan Humility' were unsuccessful, so I searched the web in the hope that somewhere somebody might have said something about what Lewis says in that paper. This is how I came across Paul Mainwood's BPhil Thesis Properties, Permutations and Physics (PDF). It's a very good thesis and contains (in section 4) an extended discussion of some of the problems I'm struggling with.
These problems all result from the assumption that perfectly natural properties do not have their causal role essentially. (I think it was Armstrong, not, as Paul says, Robert Black who dubbed this assumption 'Quidditism'. But the lousy library where I spend my days has neither Armstrong's book nor the AJP, so I might be wrong.) It follows for example that in some worlds, indistinguishable from our world, physicists get the laws of nature entirely wrong just because unlike hereabout the predicates of their best theory fail to pick out the natural properties. Similarly, in some indistinguishable worlds 'rabbit' denotes rabbits on Mondays and cherries on other days because by the weird distribution of fundamental properties at those worlds that class of things happens to be more eligible than the intrinsically gruesome class of things playing the rabbit role.
These consequences are counterintuitive and worrying, but I don't think they refute Lewis' position. (Unlike the problems I've been talking about recently.) For they have too much in common with well-known sceptical arguments: Compare
How can we be confident that our scientists find the real laws if there are ever so many possible scientists in exactly the same evidential situation who get the laws wrong?
How can we be confident that the future will resemble the past if there are ever so many people in exactly the same evidential situation but where the future doesn't resemble the past at all?
How can we be confident that we are not brains in vats if there are ever so many people in exactly the same evidential situation who are brains in vats?
Another interesting thing I learned from Paul's thesis is that something very much like Quidditism is being discussed in the context of Einstein's Hole Argument in the philosophy of physics. Several articles in the recently published Festschrift for John Stachel, Revisiting the Foundations of Relativistic Physics seem particularly relevant here. (At least one of them, "Indiscernibles, General Covariance and Other Symmetries" by Simon Saunders is available online (PDF).)
Oh, and here is Paul Mainwood's blog, though unfortunately it looks like it won't win the price for the most long-standing blog ever.