Modal Realism

Table of Contents

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)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)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)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.

Lewis #

This is going to be a page by page summary of On the Plurality of Worlds with some running commentary. Overall impressions go at the end. I’m aware that OPW was not Lewis’s only writing on the ontology of possible worlds, but since he seems to repudiate major pieces of Counterfactuals (1973), I assume that he intended OPW (1986) to stand as the more authoritative account of his views.

Summary #

Ch 1: Theoretical rewards of accepting modal realism

§1.6 is about what separates possible worlds from one another and inversely, what makes it the case that some things are worldmates of each other. Lewis’s answer is that two entities are worldmates just in case they are spatiotemporally related. That is, if there is a fact about how far apart the two entities are, or if there is a fact about how much time elapses between them, then the two objects have to be worldmates. Interesting hiccup—what about possible worlds whose spacetimes are structurally unlike ours? Newton believed that spacetime was nonrelativistic, and most people pre-theoretically believe the same today. So on the principle that everything conceivable is possible, it seems that nonrelativistic worlds are possible. But events within these worlds cannot be spatiotemporally related in the same way that events within the actual world are spatiotemporally related. Thus Lewis’s principle risks fracturing these worlds into clouds of disconnected events.

His response isn’t particularly graceful. The idea is that events in Newton-world are related to each other in an analogous way to how events in our world are related. There is a natural, pervasive, discriminating, and external relation that applies to them. How do we know there couldn’t be possible worlds so poor in structure that no such relation applies to their parts? Also, “what natural external relations could there be besides the (strictly or analogically) spatiotemporal relations?” Good question. An external relation is one that does “not supervene on the intrinsic natures of the relata taken separately, but only on the intrinsic character of the composite of the relata.” So in other words, the relation can change without the instrinsic properties of any of the relata changing. I too have a hard time imagining what this could be besides a spatiotemporal relation.

I think that virtual objects may provide another counterexample to Lewis’s analysis of worldmates as spatiotemporally related entities. (4)This point is directly inspired by Chalmers, Reality + In weather simulations, there are simulated clouds. These clouds are not spatiotemporally related to me. It makes no sense to ask about the spacetime interval between me and and the simulated clouds. Assuming that I am actual, does this mean that the clouds are merely possible? That seems wrong because all properties of the simulated entities supervene on properties of a computer somewhere in the actual world, and surely parts of distinct worlds can’t be connected by a supervenience relation.

§1.7 is about whether merely possible worlds should be called “concrete” or “abstract.” Lewis says they’re concrete. I say who cares? I’m not aware of any important issue that hinges on the concrete/abstract distinction, and I suspect it of being fake. Lewis presents the four Ways that this distinction has typically been explicated. (1) “The Way of Example: concrete entities are things like donkeys and puddles and protons and stars, whereas abstract entities are things like numbers.” (2) “The Way of Conflation: the distinction between concrete and abstract entities is just the distinction between individuals and sets, or between particulars and universals, or perhaps between particular individuals and everything else.” (3) “The Negative Way: abstract entities have no spatiotemporal location; they do not enter into causal interaction; they are never indiscernible one from another.” (4) “The Way of Abstraction: abstract entities are abstractions from concrete entities.”

I don’t find any of these Ways very appealing. Lewis points out that (1) is only as useful as our grip on the ontological status of numbers is firm. Also, think about the operational consequences of taking (1) as your definition. The way you determine whether an object is concrete or abstract is to assess whether it’s more like a donkey or more like a number. Come on. That’s a game, not respectable metaphysics. (2) analyses abstracta away by defining them to be sets or universals. Okay, but then why talk about abstracta at all? On (3), it seems like concreta and abstracta fail to cover the space of all that could be. What about non spatiotemporal Cartesian souls that nevertheless have causal effects? (4) is just very unclear. What does it mean to “subtract specificity” from, eg, a donkey?

Ch 2: Actualist objections

§2.6 Are the psychological consequences of believing in modal realism livable? There are at least two distinct worries here, one about indifference and another about sympathetic overload (not Lewis’s term).

If everything that can possibly happen does happen, in what sense do my choices matter? When I choose to do good rather than ill, I make a better world actual than would have been actual had I chosen otherwise, but I do nothing to change the balance of good and evil in the ensemble of all possible worlds. The worlds where I instead choose ill remain equally real, albeit unactualised. This is like the problem of infinitarian paralysis only worse. When you do a good deed in an infinite world, you have at least improved matters locally, even if you haven’t improved them globally. When you do a good deed in Lewis’s multiverse, it’s unclear whether you’ve done anything of even local ethical significance.

The sympathetic overload concern is that the modal realist has to believe that every possible crime is actually committed, that every possible disaster actually befalls someone. Assuming that you care about all of these misfortunes, you will find your sense of sympathy overwhelmed.

Lewis’s response, which I endorse, is that our preferences generally don’t have unrestricted scope. When I wish that people should treat each other kindly, I mean to restrict my wish to actual people. I want good things to happen at the world where I am, not just for them to happen somewhere out in logical space. I don’t think human moral psychology fully rules out unrestricted preferences, but Lewis is right that there would be something “idle” about them. “The character of the totality of all the worlds is not a contingent matter.” You might as well have a preference that the thousandth digit of π be three. (5)It isn’t, by the way. It is not now possible, nor was it ever possible for any event to satisfy your preference. There’s no way you can act on your preference either.

I’m not sure Lewis says enough about the sympathetic overload issue. He just states that he sees no reason to bemoan the evils of other worlds, which is quite plauible on a consequentialist view—see the point about idleness above. But you might also think that sympathy and mourning is just the appropriate response to evils no matter whom they befall and where. (6)For example, many people think it’s appropriate to pray for those suffering hardship while not thinking that prayer is efficacious to relieve this suffering. If someone holds this value, don’t they have a genuine reason to bemoan merely possible atrocities?

§2.8 is about the incredulous stare. How badly does MR really offend against common sense? Lewis imagines asking a “spokesman for common sense” (1) whether there exist infinitely many donkeys and (2) whether there actually exist infinitely many donkeys. MR says that the answer to (1) is yes, and the answer to (2) is no, at least assuming the actual universe is finite. On the other hand, the spokesman will find these questions quite confusing and fail to see an important difference between them. Point taken. MR is drawing a distinction between the real and the actual, a distinction that common sense just doesn’t observe. It’s not that the man in the street confidently believes real implies actual. Rather, he vaguely assumes the two concepts are synonymous, and thus sees no need to form an explicit belief about their logical relationship.

Ch 3: Ersatzisms

New idea: all that the modal logician really needs to ground their machinery is a sufficiently rich collection of consistent sets of facts. Then $\Diamond$ means that the following sentence is true relative to at least one set in the collection, and $\Box$ means it’s true relative to all such sets. Now, a possible world certainly defines a consistent set of facts, but there are other more ontologically conservative ways to get the job done. What if instead of positing that there are infinitely many equally real territories, we just posit infinitely many maps?

What kind of maps? They could be literal scale maps, leading to the view Lewis calls “pictorial ersatzism”. They could be long novels written in a semi-idealized language. This is Richard Jeffrey’s proposal, and Lewis calls is “linguistic ersatzism.” (7)Yes, the same Jeffrey who proved the Representation Theorem—in the same book no less. The maps could also be some other kind of underspecified representiational medium (magical ersatzism, Lewis’s disparaging catch-all).

Reactions #

First on the style—Who says analytic philosophy has to be dry and ponderous? Almost every page of OPW offers some witty turn of phrase or insightful metaphor, making even the most technical parts pleasant to read. There are also some delightfully bizarre passages sprinkled throughout, such as this one on the sources of our donkey knowledge:

Maybe causal acquaintance with donkeys themselves is not required—we need no backward causation to know that there will be donkeys in the next century—but in that case we are causally acquainted with the past causes of future donkeys. In the case of other-worldly donkeys, though, we can no more be acquainted with the donkeymakers than with the donkeys themselves.

In context, these sentences are all perfectly sane and meaningful, so you nod along as you read through them, then rub your eyes and laugh. (8)The only other book that’s given me this surreal reading experience was Stephen Yablo’s Aboutness. From pg 36 “The leg and the table are carrying on in the same sort of way as Saturday and the weekend.”

I used to think that “realism” had to be the most horribly overloaded term in philosophy. It seemed sloppy and confusing to call anyone who believes in Platonic ideals, or in objective normative facts, or in numbers, or in merely possible worlds a “realist.” But I’ve come to think this overloading isn’t that much of a problem, since all of the many realisms kind of stand or fall together. Most of the strong objections against one realism—eg, naturalistic debunking arguments, appeals to parsimony, concerns about epistemic access—also bite the others, and most of the compelling defenses of one realism—indispensibility arguments, appeals to theoretical elegance—can easily be adapted to support the others.

Questions #

Reading list #

Last updated 18 December 2024