Online Papers - Brian Weatherson

David Lewis 78
In the first chapter of Plurality Lewis takes a much more Quinean orthodox line.
He argues, at great length, that the best version of many philosophical theories requires
quantification over possibilities. In traditional terms, he offers an extended
indispensibility argument for unactualised possibilities. But traditional terms are
perhaps misleading here. Lewis does not say that possibilities are absolutely indispensible,
only that they make our philosophical theories so much better that we
have sufficient reason to accept them.
There are four areas in which Lewis thinks that possible worlds earn their keep.
Modality Traditional treatments of modal talk in terms of operators face several difficulties.
They can’t, at least without significant cost, properly analyse talk
about contingent existence, or talk about modal comparatives, or modal supervenience
theses. All of these are easy to understand in terms of quantification
across possibilities.
Closeness Our best theory of counterfactuals, Lewis’s theory, relies on comparisons
between possible worlds. Indeed, it relies on comparisons between this world
and other worlds. Such talk will be hard to paraphrase away if worlds aren’t
real.
Content Lewis argues, in part following Stalnaker (1984), that our best theory of
mental and verbal content analyses content in terms of sets of possibilities.
This, in turn, requires that the possibilities exist.
Properties We often appear to quantify over properties. The modal realist can take
properties to be sets of possibilia, and take such quantification at face value.
In his discussion of properties here, Lewis expands upon his theory of natural
properties that he introduced in “New Work for a Theory of Universals”, and
that we discussed in section 3.
After arguing that we are best off in all these areas of philosophy if we accept unactualised
possibilities, Lewis spends the rest of chapter 1 saying what possible worlds
are on his view. He isn’t yet arguing for this way of thinking about possible worlds;
that will come in chapter 3. For now he is just describing what he takes to be the
best theory of possible worlds. He holds that possible worlds are isolated; no part
of one is spatio-temporally related to any other world. Indeed, he holds that lack of
spatio-temporal relation (or something like it) is what marks individuals as being in
different worlds. So his theory has the somewhat odd consequence that there could
not have been two parts of the world that aren’t spatio-temporally connected. He
holds that worlds are concrete, though spelling out just what the abstract/concrete
distinction comes to in this context isn’t a trivial task. And he holds that worlds are
plenitudinous. There is a world for every way things could be. And worlds satisfy a
principle of recombination: shape and size permitting, any number of duplicates of
any number of possible things can co-exist or fail to co-exist.
6.2 Paradox in Paradise?
Chapter 2 deals with several objections to modal realism. Some of these objections
claim that modal realism leads to paradox. Other objections claim that it undermines
our ordinary practice. We will look at two examples of each.