Online Papers - Brian Weatherson

David Lewis 80
6.3 Paradise on the Cheap?
In chapter 3 Lewis looks at the alternatives to his kind of modal realism. He takes
himself to have established that we need to have possible worlds of some kind in our
ontology, but not that these possible worlds must be concrete. In particular, they can
be abstract, or what he calls “ersatz” possible worlds. Lewis does not have a single
knock-down argument against all forms of ersatzism. Instead he divides the space of
possible ersatzist positions into three, and launches different attacks against different
ones.
Lewis starts with what he calls “linguistic ersatzism”. This is the view that ersatz
possible worlds are representations, and the way they represent possibilities is something
like the way that language represents possibilities. In particular, they represent
possibilities without resembling possibilities, but instead in virtue of structural features
of the representation.
He levels three main objections to linguistic ersatzism. First, it takes modality as a
primitive, rather than reducing modality to something simpler (like concrete possible
worlds). Second, it can’t distinguish qualitatively similar individuals in other possible
worlds. Lewis argues that will mean that we can’t always quantify over possibilia, as
we can in his theory. Third, it can’t allow as full a range of ‘alien’, i.e. uninstantiated,
natural properties as we would like. Sider (2002) has replied that some of these challenges
can be met, or at least reduced in intensity, if we take the pluriverse (i.e. the
plurality of worlds) to be what is represented, rather than the individual worlds.
The second theory he considers is what he calls “pictoral ersatzism”. This is the
view that ersatz possible worlds are representations, and the way they represent possibilities
is something like the way that pictures or models represent possibilities. That
is, they represent by being similar, in a crucial respect, to what they are representing.
The pictoral ersatzist, says Lewis, is caught in something of a bind. If the representations
are not detailed enough, they will not give us enough possibilities to do the
job that possible worlds need to do. If they are detailed enough to do that job, and
they represent by resembling possibilities, then arguably they will contain as much
problematic ontology as Lewisian concrete possible worlds. So they have the costs of
Lewis’s theory without any obvious advantage.
The final theory he considers is what he calls “magical ersatzism”. Unlike the previous
two theories, this theory is defined negatively. The magical ersatzist is defined
by their denial that possible worlds represent, or at least that they represent in either
of the two ways (linguistic and pictoral) that we are familiar with. And Lewis’s primary
complaint is that this kind of theory is mysterious, and that it could only seem
attractive if it hides from view the parts of the theory that are doing the philosophical
work. Lewis argues that as soon as we ask simple questions about the relationship
that holds between a possibility and actuality if that possibility is actualised, such as
whether this is an internal or external relation, we find the magical ersatzist saying
things that are either implausible or mysterious.
It isn’t clear just who is a magical ersatzist. Lewis wrote that at the time he wrote
Plurality no one explicitly endorsed this theory. This was perhaps unfair to various
primitivists about modality, such as Adams (1974), Plantinga (1974) and Stalnaker