Modal Realism

Some apparently sane philosophers believe that all possible worlds exist. In other words, they deny that real implies actual. Why would anyone believe this view, which is called modal realism?

(1) The most natural way of interpreting statements about chance is possibilist. Every claim about a chance is actually a claim about the distribution of possible worlds where some proposition is true. So when I say

The chance of event $E$ is $p$ ,

the possibilist interpretation(1) (1)This is my recollection of what Handfield calls the “modal volumes interpretation.” Exact ref and fact checking needed. would be

If $\Omega$ is the set of all possible worlds consistent with my evidence, and $\mu$ is some appropriate measure defined over sets of possible worlds, then \[\mu\left(\{w \in \Omega| E \text{ occurs in } w\}\right)/\mu(\Omega) = p.\]

Of course, you can criticize possibilist interpretations on a number of fronts, but suppose you do accept that claims about chance are really claims about possible worlds. This then sets up an indispensability argument since you can’t articulate the theory of statistical mechanics without making claims about chance, and stat mech is our best scientific explanation for thermodynamics. So either you give up on IBE, you deny that stat mech requires claims about chance, or you become a modal realist.

(2) Certain puzzles in semantics and the philosophy of language disappear if you accept that possible worlds are real. For instance, when I say “I could have been an astronaut,“(2) (2)By the way, grammarians call the word could a modal auxiliary verb, which is the same modal as in modal realism. it’s not totally clear what could make this statement true or false. You might try to analyse my statement as “There are possible worlds in which the being most like me is an astronaut,” but this move doesn’t work unless possible worlds are real. My original sentence pertained only to reality, so surely properties of non-existent worlds can’t make such a statement true. Hence modal realism.

Lewis gives a few other semantic arguments along similar lines in OPW. I haven’t looked into their specifics yet, largely because I don’t find the form of argument very persuasive. We shouldn’t expand our ontology to fit our language, but rather the other way around. Suppose you find that ordinary people are in the habit of asserting a proposition that doesn’t mean anything unless Santa Claus exists. Further, let’s assume that we have no empirical evidence for Santa’s existence.(3) (3)A highly unrealistic assumption Then I think the correct response is not to start believing in Santa. Instead, you should just accept that people say all kinds of crazy stuff, and sometimes it doesn’t make sense. Absent some very good reason to think your statements are robustly true—eg, they’re part of a well-confirmed scientific theory—you don’t get to contort your view of reality to fit your habits of speech.

Getting back to the modal realism debate, maybe counterfactuals are just silly and have no truthmakers. Maybe modal talk doesn’t mean anything. I think these views are much more plausible than the view that there are infinitely many real but non-actual worlds whose properties make counterfactuals true or false.

Questions #

How sound is the physical indispensibility argument for MR I sketched out above? If it’s unsound, is there a better version?
Should I take semantic arguments for MR more seriously?
Does MR have any practical implications? Are they liveable?

Reading list #

Lewis, On the Plurality of Worlds. The classic defense of modal realism.
Handfield, A Philosophical Guide to Chance ✔ discusses whether the best interpretations of probability commit us to modal realism.
Sinhababu, “Possible Girls.” I can’t tell whether this paper is a spoof.

Last updated 2 November 2024