Online Papers - Brian Weatherson

David Lewis 57
part of natural language and it is philosophically interesting to figure out how they
work. But counterfactuals would play a large role in Lewis’s metaphysics. Many of
Lewis’s attempted reductions of nomic or mental concepts would be either directly
in terms of counterfactuals, or in terms of concepts (such as causation) that he in turn
defined in terms of counterfactuals. And the analysis of counterfactuals, which uses
possible worlds, would in turn provide motivation for believing in possible worlds.
We will look at these two metaphysical motivations in more detail in section 4, where
we discuss the relationship between counterfactuals and laws, causation and other
high-level concepts, and in section 5, where we discuss the motivations for Lewis’s
modal metaphysics.
3.1 Background
To the extent that there was a mid-century orthodoxy about counterfactual conditionals,
it was given by the proposal in Nelson Goodman (1955). Goodman proposed
that counterfactual conditionals were a particular variety of strict conditional.
To a first approximation, If it were the case that p, it would be the case that q (hereafter
p � q) is true just in case Necessarily, either p is false or q is true, i.e. �(p ⊃
q). Goodman realised that this wouldn’t work if the modal ‘necessarily’ was interpreted
unrestrictedly. He first suggested that we needed to restrict attention to those
possibilities where all facts ‘co-tenable’ with p were true. More formally, if S is the
conjunction of all the co-tenable facts, then p � q is true iff �((p ∧ S) ⊃ q).
Lewis argued that this could not be the correct set of truth conditions for p �
q in general. His argument was that strict conditionals were in a certain sense indefeasible.
If a strict conditional is true, then adding more conjuncts to the antecedent
cannot make it false. But intuitively, adding conjuncts to the antecedent of a counterfactual
can change it from being true to false. Indeed, intuitively we can have long
sequences of counterfactuals of ever increasing strength in the antecedent, but with
the same consequent, that alternate in truth value. So we can imagine that (3.1) and
(3.3) are true, while (3.2) and (3.4) are false.
(3.1) If Smith gets the most votes, he will be the next mayor.
(3.2) If Smith gets the most votes but is disqualified due to electoral fraud, he will be
the next mayor.
(3.3) If Smith gets the most votes, but is disqualified due to electoral fraud, then
launches a military coup that overtakes the city government, he will be the
next mayor.
(3.4) If Smith gets the most votes, but is disqualified due to electoral fraud, then
launches a military coup that overtakes the city government, but dies during
the coup, he will be the next mayor.
If we are to regard p � q as true iff �((p ∧ S) ⊃ q), then the S must vary for
different values of p. More seriously, we have to say something about how S varies
with variation in p. Goodman’s own attempts to resolve this problem had generally
been regarded as unsuccessful, for reasons discussed in Bennett (1984). So a new
solution was needed.