Online Papers - Brian Weatherson

David Lewis 66
of a belief is a property. To believe something then is to locate yourself within a
class of possible individuals; to believe that you are one of the individuals with the
property. More simply, beliefs are the self-ascriptions of properties.
Within this framework, it is easy to resolve the puzzles we addressed at the top of
the section. If I believe that BW is a crook, I self-ascribe the property of inhabiting
a world in which BW is a crook. (On Lewis’s theory, beliefs that are not explicitly
self-locating will be beliefs about which world one is in.) If I believe I am not a crook,
I self-ascribe the property of not being a crook. Since there are possible individuals
who are (a) not crooks but (b) in worlds where BW is a crook, this is a consistent
self-ascription. Indeed, I may even have strong evidence that I have both of these
properties. So there is no threat of inconsistency, or even irrationality here.
Lewis’s suggestion about how to think of self-locating mental states has recently
been very influential in a variety of areas. Adam Elga (2000a, 2004a) has extensively
investigated the consequences of Lewis’s approach for decision theory. Andy Egan
(2007a) has developed a novel form of semantic relativism using Lewis’s approach
as a model. Daniel Nolan (2007) has recently argued that Lewis’s approach is less
plausible for desire than for belief, and Robert Stalnaker (2008a) argues that the view
makes the wrong judgments about sameness and difference of belief across agents and
times.
4.6 Natural Properties
One classic problem for interpretationism is that our dispositions massively underdetermine
contents. I believe that (healthy) grass is green. But for some interpretations
of ‘grue’, ascribing to me the belief that grass is grue will fit my dispositions just as
well. As Lewis points out towards the end of “New Work For a Theory of Universals”
(1983c), if we are allowed to change the interpretations of my beliefs and desires
at the same time, the fit can be made even better. This looks like a problem for
interpretationism.
The problem is of course quite familiar. In different guises it is Goodman’s
grue/green problem, Kripkenstein’s plus/quus problem, Quine’s gavagai problem,
and Putnam’s puzzle of the brain in a vat with true beliefs (Goodman, 1955; Wittgenstein,
1953; Kripke, 1982; Quine, 1960; Putnam, 1981). One way or another it
has to be solved.
Lewis’s solution turns on a metaphysical posit. Some properties, he says, are
more natural than others. The natural properties are those that, to use an ancient
phrase, carve nature at the joints. They make for objective resemblance amongst the
objects that have them, and objective dissimilarity between things that have them
and those that lack them. The natural properties, but not in general the unnatural
properties, are relevant to the causal powers of things. Although science is in the
business of discovering which natural properties are instantiated, when Lewis talks
about natural properties he doesn’t mean properties given a special role by nature. It
is not a contingent matter which properties are natural, because it isn’t a contingent
matter which properties make for objective similarity.
Some properties are perfectly natural. Other properties are less natural, but not
all unnatural properties are alike. Green things are a diverse and heterogeneous