The Surprisingly Robust Case For Modal Realism

EDIT: This post is nonsense for reasons I'll explain in the future. Modal realism is almost certainly false.

(I have now thought more about this and think modal realism has insurmountable problems leading to a near zero credence in it. Nevertheless, I’ll keep this up just to see my pre reflection thoughts about it. )

Modal realism says that everything metaphysically possible actually happens. There’s some possible world in which everything that is metaphysically possible actually occurs. Currently my credence in modal realism is about 8%—I think there are some really good arguments for modal realism. Here I’ll present several. Note, I haven’t read much literature around modal realism—so take my thoughts with a grain of salt. I think it’s helpful to think about an idea for a while before reading the literature about it.

1 Lewis’ primary argument for modal realism related to it solving modal questions. Much like it’s reasonable to posit mathematical entities to explain mathematical truths, or moral facts to explain moral truths, given the relevance of possible worlds, it’s useful to posit possible worlds. Lewis had some very complicated arguments along these lines that are way above my paygrade, but there is some argument here. However, on first blush, I don’t find it very persuasive. It seems like if we posit two possible scenarios

A) Other worlds actually exist.

B) They don’t.

It’s not clear how the modal analysis would be different between the two of them. This is because the possible worlds are causally inert and don’t interact with the actual world. Lewis had responses to this, but having not examined them in much detail, I don’t find it very persuasive.

2 There’s a compelling anthropic argument for modal realism. If modal realism is true, the probability that I would exist is exactly 100%. Thus, modal realism provides a satisfactory explanation of why conscious life arises, which seems otherwise very improbable.

3 Modal realism explains why we are conscious. Those who aren’t conscious are obviously not aware of that fact.

4 Modal realism explains the existence of things to which morality applies. If there are worlds in which there is no happiness or suffering, and thus nothing to which morality applies, then the people in those worlds aren’t complaining, because they’re not aware of it. Thus, modal realism explains why there are the natural facts which moral facts supervene on.

5 Modal realism can also explain examples of weird events that we would only be aware of in which they happened. E.G. suppose that the resurrection evidence about Jesus will occur in one in a million worlds roughly like ours. On modal realism, there will be infinite worlds in which Jesus exists and we have similar evidence. Thus, the P(Jesus Resurrection Evidence|Modal Realism) = 100%. While perhaps we think that this would not be evidence, the anthropic principle would make it evidence, because in the absence of Jesus we wouldn’t even consider the possibility. I’m very uncertain about this argument, though it might have some force.

6 Modal realism explains the data appealed to by the Kalam cosmological argument. A universe beginning to exist is metaphysically possible and so it would exist. Ditto four a four dimensional block universe.

7 Modal realism has a nice feel of theoretical elegance. While assessing the simplicity of a theory is difficult, there is something simple seeming about the theory of everything for ultimate reality being the following.

(If X is possible X is real).

8 There are possible modal ontological arguments for modal realism. Modal realism would be a theory of all of modality, so it seems like it is the type of thing that could be necessary. At least possibly necessary.

9 If we think that limits on possibility need an explanation, which seems somewhat intuitive—we were rational to believe there was no cosmic speed limit before discovering the speed of light and in the absence of something being made impossible, the thing is possible—this would entail modal realism.

10 One prediction modal realism would seem to make is that the universe is infinitely large, because 100% of observers would be in infinitely large universes. This seems somewhat plausible.

11 Modal realism seems to predict that our universe would be exceptional in how much life it creates. It doesn’t seem to be, so this does seem to count against modal realism, but it’s not clear.

I’ll now read some of Lewis’ writings on it and probably write another article about my conclusions.