Online Papers - Brian Weatherson

David Lewis 77
Lewis held that this world was just one among many like it. A proposition, p is
possibly true if and only if p is true in one of these worlds. Relatedly, he held that
individuals like you or I (or this computer) only exist in one possible world. So what
it is for a proposition like You are happy to be true in another world is not for you to
be happy in that world; you aren’t in that world. Rather, it is for your counterpart to
be happy in that world.
Lewis wrote about modal realism in many places. As early as Counterfactuals he
wrote this famous passage.
I believe, and so do you, that things could have been different in countless
ways. But what does this mean? Ordinary language permits the
paraphrase: there are many ways things could have been besides the way
they actually are. I believe that things could have been different in countless
ways; I believe permissible paraphrases of what I believe; taking the
paraphrase at its face value, I therefore believe in the existence of entities
that might be called ‘ways things could have been.’ I prefer to call them
‘possible worlds.’ (1973b, 84)
And Lewis used counterpart theory throughout his career to resolve metaphysical
puzzles in fields stretching from personal identity (“Counterparts of Persons and
Their Bodies” (1971b)) to truthmaker theory (“Things qua Truthmakers” (2003)). Indeed,
Lewis’s original statement of counterpart theory is in one of his first published
metaphysics papers (“Counterpart Theory and Quantified Modal Logic” (1968)).
But the canonical statement and defence of both modal realism and counterpart
theory is in On the Plurality of Worlds (1986b), the book that grew out of his 1984
John Locke lectures. This section will follow the structure of that book.
The little ‘argument by paraphrase’ from Counterfactuals is a long way from an
argument for Lewis’s form of modal realism. For one thing, the argument relies on
taking a folksy paraphrase as metaphysically revealing; perhaps we would be better
off treating this as just a careless manner of speaking. For another, the folksy paraphrase
Lewis uses isn’t obviously innocuous; like many other abstraction principles
it could be hiding a contradiction. And the argument does little to show that other
possible worlds are concreta; talking of them as ways things could be makes them
sound like properties, which are arguably abstracta if they exist at all. The first three
chapters of Plurality address these three issues. The fourth chapter is an extended discussion
of the place of individuals in modal realism. We’ll look at these chapters in
order.
6.1 A Philosophers’ Paradise
The short argument from Counterfactuals that I quoted seems deeply unQuinean.
Rather than saying that possible worlds exist because they are quantified over in the
best paraphrase of our theories, Lewis says they exist because they are quantified
over in just one paraphrase of our theories. To be sure, he says this is a permissible
paraphrase. On the other hand, there is vanishingly little defence of its permissibility.