Confessions of a Modal Realist, Part 2

by Leonard Finkelman

Part of the following story is false. You probably would have figured that out on your own. Nevertheless, I pride myself on my honesty as much as on my irrational need to force dinosaur references on my readers.

My final drive down to the University of Virginia before graduation almost wasn’t memorable. The weather was drab, the traffic was moderate, and my third run through U2’s “All That You Can’t Leave Behind” hadn’t produced any outstanding singalong performances. It was somewhere around Warrenton, Virginia that I first saw a bald eagle on the side of the road. I remember thinking it was unusual. When I saw a second bald eagle twenty miles further down the road, I thought it was noteworthy. When a Tyrannosaurus rex rushed through the treeline beside the road, snatched the second eagle in its mouth, and disappeared back into the foliage, I couldn’t manage any coherent thoughts at all. When my internal monologue returned, it began, “This isn’t the right ecology for a twenty-five-foot-long warm-blooded predator.” [1]

Previously on “Confessions of a Modal Realist,” I confessed to being a modal realist. (Aptly titled!) Modal realism is the view that there really exists an alternate universe for every true counterfactual proposition. So you might think that I’m committed to saying that somewhere out in the vastness of all times and spaces someone named “Leonard Finkelman” really saw a T. rex eat a bald eagle. You probably think that’s preposterous. You’d be correct about the second point, but incorrect about the first. I’m going to explain myself and defend a modified form of modal realism, but first I’ll have to go on an informative digression.

Is it possible that I could see a T. rex? I have no idea. I can’t say one way or the other. Neither can you. You see, the question, “Is it possible?” is ambiguous. Philosophers recognize (at least) three kinds of possibility: physical, logical, and metaphysical [2].

Without any training in philosophy, people tend to say that fantastical stories like the one above are impossible. T. rex went extinct 66 million years ago. Of course I couldn’t have seen one on a Virginia roadside. Some kind of space-time anomaly would be required to make my story true, and since those don’t happen around here without anyone noticing, my story has to be false. This response uses the laws of nature as the standard of possibility: something is possible only if physics allows it. That’s physical possibility.

Intro Philosophy students are often incredulous when I insist that it is truly possible that I could see a T. rex on the side of the road, or fly like Superman, or use mental power alone to move a remote control across a room. They’re incredulous because our day-to-day lives demand that we consider possibilities against the physical standard. But none of those crazy propositions is self-contradictory, which (literally) means that they are not always false. There are conditions — however far-fetched — under which they are true. The propositions are therefore logically possible, if not physically possible.

Anything that is physically possible is also logically possible, but not everything that is logically possible is physically possible. So far, so good.

But then there’s metaphysical possibility. Excuse me for a moment while I take a preparatory dose of Advil [3].

Here is a textbook definition: a proposition is metaphysically possible if and only if there is a possible world on which the proposition is true. Here is a textbook example: the proposition “Water is not H2O” is logically possible, but metaphysically impossible. “Water” does not mean “H2O,” but the particular arrangement of hydrogen and oxygen atoms in our universe gives water its particular features. The proposition is not self-contradictory, but it’s false on all possible worlds. This serves to demonstrate why students are so anxious to sell their textbooks back to the bookstore, even at a severe discount, at the end of the semester.

Hilary Putnam argued for this account of metaphysical possibility in a famous 1973 paper [4]. Philosophers may be more or less sympathetic to his view. For my part, I’m worried that there’s some question-begging going on. What exactly does “water” mean? Is the chemical compound H2O, when found in our universe, the only substance that can have all the features associated with water? Considering these questions carefully, I find it difficult to distinguish metaphysical possibility from the other kinds, and so it’s difficult to know what makes a possible world possible.

Putnam argues that “water is not H2O” is not self-contradictory because the word “water” was used before anyone knew about hydrogen or oxygen. Fair enough. But sometimes we use words without realizing what those words logically commit us to accepting. For example: is the statement “Leonard Finkelman saw a T. rex” self-contradictory or not? Tyrannosaurus rex lived and died in an environment whose atmospheric oxygen content was 50% higher than our own; maybe humans are only viable in a developmental environment different from a T. rex’s, in which case no living specimen of one could ever see a living specimen of the other. And even if the biological difficulties were overcome, there’s a logical problem: “Tyrannosaurus rex” names a breeding population of organisms that died out millions of years ago, and so nothing living today could possibly be a T. rex (let’s assume — fairly! — that necrophilia does not constitute breeding) [5]. Maybe part of what makes me who I am is a set of conditions that precludes extant T. rexes. Propositions that don’t seem self-contradictory at first glance may not pass more rigorous logical tests. Maybe “water” really did mean “H2O” all along and we only just recently became aware of that fact [6]. Maybe the reason that there are no possible worlds on which water isn’t H2O is that the proposition is in fact self-contradictory.

This all assumes that H2O is the only molecule that can produce the features of water, of course, and maybe that didn’t have to be the case. But maybe it did. One of the more important debates in philosophy of science is whether or not the laws of nature could have been different. Certainly, I can imagine a possible world with (say) a different Planck Constant. But reality itself cannot care less about my a priori speculations. We only have the actual world to work with, and so the question of whether or not laws of nature are necessary is necessarily moot. So maybe there aren’t any possible worlds on which water isn’t H2O because the laws of nature are in fact logically necessary.

Here’s my point: we need a standard for recognizing possible worlds. Is that standard different from the logical standard? Maybe. Is it different from the physical standard? Perhaps. Heck, is the physical standard really different from the logical standard? I don’t know. The most honest answer to any question about possible worlds is that I just don’t know.

For too long, discussions of possibility have relied on a priori intuition-mongering. As we well know, however, intuitions aren’t exactly reliable indicators of truth. Some philosophers — most notably, Peter van Inwagen — therefore endorse modal skepticism, or the view that the truth or falsity of counterfactual propositions can’t be known. I agree. If a priori speculation is insufficient to know what’s possible and what isn’t, then we also need a posteriori observation.

This is why I’m a modal realist. Is it possible that I could have seen a T. rex? All I know is that I never did. I can rearrange various aspects of my experiences to create what seems like a coherent story — as modal fictionalists suggest — about seeing an extinct animal, but that’s no guarantee of a logically consistent state of affairs [7]. I don’t make counterfactual propositions true. Neither does the actual world [8]. But if reality includes an alternate world on which the counterfactual proposition is factual, that would certainly demonstrate the proposition’s logical consistency. If counterfactual propositions are true at all, then other worlds would be the best candidates to make them true.

I’ll admit that this is a different view from traditional modal realism. As I mentioned last time, Lewis asserts that there are infinitely many possible worlds, thus making all logically consistent counterfactual propositions true. I’m not so willing to allow semantics to dictate my ontology. As I’ve argued, whether or not there are other possible worlds is an empirical question. And as an empiricist, I believe that empirical questions can only be answered by investigation of the external reality. Maybe there’s a possible world on which I saw a T. rex. Maybe there isn’t. Certainly, I can’t know one way or the other without having access to a state of affairs in which something like that does happen.

I like to think of this as modal realism naturalized. Since they aren’t true a priori, counterfactual propositions — e.g., “Leonard Finkelman saw a T. rex” — need to be made true by some external reality. If the actual universe is the only real universe, then there would be nothing to make counterfactual propositions true (remember, fictional rearrangements of states of affairs prove nothing). If there are other universes (i.e., possible worlds) in addition to ours, then whatever happens in those universes are clearly consistent possibilities. These other universes may be unobservable, but that’s okay. It’s not as if we really had a very firm grasp on possibility in the first place. And so: modal realism or bust.

Here ends my confession. 

Signed, the actual Leonard Finkelman.

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[1] You’re right: the first bald eagle was the false part.

[2] There’s also a fourth — epistemic possibility — but that really has little to do with possible worlds. I may say, for example, “There might be a Simpsons rerun airing right now,” which means that I just don’t know if the rerun is airing or not. I suppose that a philosopher could interpret that proposition using possible world semantics, but that exercise would be tortuous, silly, and completely unnecessary. I also spend a lot of my professional time talking about biological possibility. I’m afraid that I’d have to charge you for that one and my rates are unreasonable.

[3] By “dose of Advil,” I really mean “nap,” of course. I’m writing this on a Sunday afternoon.

[4] More famous, perhaps, for inspiring an episode of “Pinky and the Brain” than for making a lasting contribution to philosophy.

[5] Yes, this would also mean that “Jurassic Park” is logically impossible. That’s philosophy: crushing your dreams since 400 BCE.

[6] Putnam’s argument also ignores the fact that languages evolve as surely as species do. One charming example I came across recently is how the meaning of the term “nimrod” has changed in the last century, thanks in large part to the popularity of a mischievous lagomorph.

[7] This, after having taken four graduate-level logic courses (and two about modal logic)! Choose your course of study carefully, kids.

[8] I know that some of you are anxious to pull out a reference to quantum indeterminacy. Don’t. It doesn’t have to mean what you think it means. High-level physicists agree, and at least one sees this as a source of embarrassment.