In trying to make sense of the way we talk about possibility, David Lewis argued that everything that could possibly exist does exist, in some possible world. But our best scientific theories suggest that there are no real facts about what’s possible, and therefore possible worlds aren’t real, argues John Divers.
David Lewis (in)famously argued that just about everything you can think of really exists. Just as zebras exist, so do unicorns. Just as brains exist so do the immaterial minds that Descartes thought were associated with them. And just as there exists a universe that conforms to special relativity, so there exists a universe that conforms to Newton’s laws. What is driving all this is a pair of thoughts: (a) there are real facts about what is possible and (b) these facts consist in the real existence of things in other possible worlds. My response is that (b) is moot because we have no reason to believe that (a) is true. We have no reason to believe that there are real facts about possibility because: (i) only science could tell us that there are, and (ii) it doesn’t.
Lewis’s conception of what there is may be extraordinary, or even shocking, but it is not arbitrary. For there is a governing theoretical principle here: it is that everything that could possibly exist does exist. Nor is Lewis’s conception irrational. For there is an extended argument in support of it – in sum: genuine modal realism (as he calls his view) makes much better sense of a great deal of what we hold true than does the alternative (common-sense, conservative) conception that correlates existence with what is actual rather than with what is possible. The reader will note that for Lewis’s view to be consistent he must have been thinking that what actually exists is only a part of what exists overall. That is, indeed, what he thought.
Lewis’s conception of what there is may be extraordinary, or even shocking, but it is not arbitrary.
Lewis thinks that when we say that something actually exists, we are saying that it exists in our world (our universe). And, according to Lewis, that is a restricting thought: it is a thought about what exists locally – like saying that something exists in our country, or on our planet. At one time we may have thought that ours was the only planet. Accordingly, we would have been insensitive to the distinction between all that exists on our planet and all that exists, overall. Similarly, Lewis argues, we are inclined to be insensitive to the distinction between all that exists in our world and all that exists, overall. But when we observe that distinction it allows us to explain the real facts about possibility. Those facts are found with the existence of things in a reality that is bigger than the local part of that reality we call “actual” (our universe): the fact about what is possible are facts about the broader reality that includes all the other (non-actual) worlds as well as our (actual) world. As Graham Priest puts it, just as we often, these days, use the suffix “ism” to impute prejudice or discrimination (as in “racism”, “sexism”) so Lewis (and others) think of common-sense, conservative “actualism” - the belief that only things in our-world count as real - as metaphysically prejudiced or discriminatory. It is a philosophically unwarranted exclusion of much of what exists.
Some philosophers have been convinced by Lewis, but most have not. And for those of us – for I am one – who have not been convinced, the challenge is to show where Lewis’s argument against actualism goes wrong. I think that the argument goes wrong because it is only science that tells us what is real, and science does not tell us that there are real facts about possibility. To explain myself, I have to explain how (I think) science tells us what is real.
It is only science that tells us what is real, and science does not tell us that there are real facts about possibility.
Science tells us what's real, not what's possible
Metaphysics is the branch of philosophy that seeks to answer questions about the extent and nature of reality. And ontology is the branch of metaphysics that seeks to answer the more specific question, what exists? Now everyone agrees that science does at least some of the work of ontology. Science tells us that Higgs bosons exist, that regressive genes exist, that sociopaths exist, that pterodactyls existed and that new species will come to exist. So, let us agree, science tells us that physical objects (such as Higgs bosons etc.) exist. The question then is whether science is telling us that everything that exists is a physical object: that the only things that exist are physical objects. This latter ontological thesis is called physicalism. And I think that science is telling us that physicalism is false: science is telling us that some things other than physical objects exist. To see why, let us look at some science
The gravitational constant appears in the Einstein field equations of general relativity where Gμν is the Einstein tensor, Λ is the cosmological constant, gμν is the metric tensor, Tμν is the stress-energy tensor, and κ is a constant originally introduced by Einstein that is directly related to the Newtonian constant of gravitation: ≈ 1.866×10−26 m⋅kg−1.
There are two striking facts about this sentence. The first fact is that no term appears to refer to a physical object. The second fact is that many terms in the sentence refer to things that are not physical objects. Some of the terms refer to tensors: and in similar bits of mathematical physics we would find terms for functions, vectors, limits etc. Several of the terms refer to natural numbers: and in similar bits of mathematical physics we would find terms for negative numbers, irrational numbers and complex numbers. Now, I believe that the physics is telling us how things really are, and that we have reason to think that what we are being told is true (or at least approximately true, or that some such theory is true … but put those niceties aside). But I think, that being so, we have to accept (on current evidence) all the existential claims that physics implies. That means that we should accept the existence of physical objects, certainly, (Higgs bosons etc.) but also the existence of some mathematical entities that are not physical objects.