Online Papers - Brian Weatherson

David Lewis 75
We might wonder why Lewis found this such an interesting project. If physics teaches
that Humean supervenience is false, why care whether there are also philosophical
objections to it? There are two (related) reasons why we might care.
Recall that we said that Humean supervenience is a conjunction of several theses.
One of these is a thesis about which perfectly natural properties are instantiated in
this world, namely local ones. That thesis is threatened by modern physics. But the
rest of the package, arguably, is not. In particular, the thesis that all facts supervene
on the distribution of perfectly natural properties and relations does not appear to be
threatened. (Though see (Maudlin, 2007, Ch. 2) for a dissenting view.) Nor is the thesis
that perfectly natural properties and relations satisfy a principle of recombination
threatened by modern physics. The rough idea of the principle of recombination is
that any distribution of perfectly natural properties is possible. This thesis is Lewis’s
version of the Humean principle that there are no necessary connections between
distinct existences, and Lewis is determined to preserve as strong a version of it as he
can.
Although physics does not seem to challenge these two theses, several philosophers
do challenge them on distinctively philosophical grounds. Some of them suggest
that the nomic, the intensional, or the normative do not supervene on the distribution
of perfectly natural properties. Others suggest that the nomic, intentional,
or normative properties are perfectly natural, and as a consequence perfectly natural
properties are not freely recombinable. The philosophical arguments in favour
of such positions rarely turn on the precise constitution of the Humean’s preferred
subvenient base. If Lewis can show that such arguments fail in the setting of classical
physics, then he’ll have refuted all of the arguments against Humean superveience
that don’t rely on the details of modern physics. In practice that means he’ll have
refuted many, though not quite all, of the objections to Humean supervenience.
A broader reason for Lewis to care about Humean supervenience comes from
looking at his overall approach to metaphysics. When faced with something metaphysically
problematic, say free will, there are three broad approaches. Some philosophers
will argue that free will can’t be located in a scientific world-view, so it should
be eliminated. Call these ‘the eliminativists’. Some philosophers will agree that free
will can’t be located in the scientific world-view, so that’s a reason to expand our
metaphysical picture to include free will, perhaps as a new primitive. Call these ‘the
expansionists’. And some philosophers will reject the common assumption of an incompatibility.
Instead they will argue that we can have free will without believing in
anything that isn’t in the scientific picture. Call these ‘the compatibilists’.
As the above quote makes clear, Lewis was a compatibilist about most questions
in metaphysics. He certainly was one about free will. (“Are We Free to Break the
Laws?” (1981a).) And he was a compatibilist about most nomic, intentional and
normative concepts. This wasn’t because he had a global argument for compatibilism.
Indeed, he was an eliminativist about religion (“Anselm and Actuality” (1970a),
“Divine Evil” (2007)). And in some sense he was an expansionist about modality.
Lewis may have contested this; he thought introducing more worlds did not increase
the number of kinds of things in our ontology, because we are already committed