The infinite future problem for causal accounts of metaphysical possibility

Starting with my dissertation, I’ve defended an account of metaphysical possibility on which it is nothing other than causal possibility. I would try to define this as follows:

• p is possible₀ iff p is actually true

• p is possible_n + 1 iff things have the causal power to make it be that p is possible_n.

• p is possible iff p is possible_n for some n.

I eventually realized that this runs into problems with infinite future cases. Suppose a coin will be tossed infinitely many times, and, as we expect, will come up heads infinitely many times and tails infinitely many times. Let p be the proposition that all the tosses will be heads. Then p is false but possible. Moreover, it is easy to convince oneself that it’s not possible_n for any finite n. Possibility_n involves n branchings from the actual world, while p requires infinitely many branchings from the actual world.

This has worried me for years, and I still don’t have a satisfying solution.

But yesterday I realized a delightful fact. This problem does nothing to undercut the basic insight of my account of metaphysical possibility, namely that metaphysical possibility is causal possibility. All the problem does is undercut one initially plausible way to given an account of causal possibility. But if we agree that there is such a thing as causal possibility, and I think we should, then we can still say that metaphysical possibility is causal possibility, even if we do not know exactly how to define causal possibility in terms of causal powers.

(There is one danger. Maybe the true account of causal possibility depends on metaphysical possibility.)

Might well

It’s occurred to me that the “might well happen that” operator makes for an interesting modality. It divides into an epistemic and a metaphysical version. In both cases, if it might well happen that p, then p is possible (in the respective sense). In both cases, there is a tempting paraphrase of the operator into a probability: on the epistemic side, one might say that it might well happen that p if and only if p has a sufficiently high epistemic probability, and on the metaphysical side, one might say that it might well happen that p if and only if p has a sufficiently high chance given the contextually relevant background. In both cases, it is not clear that the probabilistic paraphrase is correct—there may be (might well be!) cases of “might well happen that” where numerical probabilities have no place. And in both cases, “might well happen that” seems context-sensitive and vague. It might well be that thinking about this operator could lead to progress on something interesting.

An argument against a giant multiverse

Tariq Nazeem emailed me a really cool and simple argument against certain kinds of gigantic multiverses. I’ve tweaked the argument a little, and here it is.

Start with this, as the target of the reductio ad absurdum:

1. Every metaphysically possible kind of substance exists.

But:

2. It is metaphysically possible to have a substance that has the causal propensity to turn every colorable object red every second.

(Colorable objects are things are like trees and dogs, but not numbers, photons or electromagnetic fields.)

Well, it follows from (1) and (2) that:

3. So, there is a substance that has the causal propensity to turn every colorable object red every second.

4. So, every colorable object turns red every second.

5. But I am a colorable object that does not turn red every second. (Empirical observation)

6. Contradiction!

My initial objection to Nazeem’s argument was that in a typical philosophical giant multiverse theory, the multiverses exist in separate spacetimes. (I think Nazeem’s own target was a view where they were in a single spacetime, but that is a less common view.) But then I realized that there is nothing absurd about a substance affecting things that are not spatiotemporally connected to it—classical theists think God is like that. Substances have causal propensities that specify the types of things they affect and the circumstances in which they affect them, and there is nothing absurd about a specification of these things and circumstances that makes no reference to a spatiotemporal connection. Therefore, (2) is pretty plausible, notwithstanding the fact that the colorable objects might exist in other spacetimes than the substance making them red does.

A different objection to this argument is what to say about conflict. What if reality included a substance that constantly made everything colorable red and another substance that constantly made everything colorable blue? Would I then be red all over and blue all over at the same time? But that’s impossible.

I am not completely clear on what to say to this objection.

One thought is: So much the worse for our giant multiverse—there are metaphysically possible pairs of kinds of substances, like the constant-reddenner and the constant-bluer, that simply cannot both be exemplified.

But on the other hand, maybe the possibility of conflict suggests that when we fully specify the causal propensities of a substance, we need to specify how they would interact with other causal propensities. Thus, we might have a constant-reddener that in the absence of other color-setters turns everything red, but in the presence of a constant-bluer, turns everything purple. However, it seems metaphysically possible to also have an overriding-reddener which makes everything red notwithstanding whatever other things exist. Then there could also be an overriding-bluer which makes everything blue notwithstanding whatever other things exist. And again this refutes (1).

Another intuition concerning the Brouwer axiom

Suppose the Brouwer axiom is false. Thus, there is some possible world w₁ such that at w₁ our world w₀ is impossible. Here’s another world that’s impossible at w₁: the world w_⊥ at which every proposition is both true and false. Thus at w₁, possibility does not distinguish between our lovely world and w_⊥. But that seems to me to be a sign that the possibility in question isn’t making the kinds of distinctions we want metaphysical modality to make. So, if we are dealing with metaphysical modality, the Brouwer axiom is true.

You can't run this argument if you run this one.

Might all infinities be the same size?

A lot of people find Cantor's discovery that there are different infinities paradoxical. To be honest, there are many counterintuitive things involving infinities, but this one doesn't strike me as particularly counterintuitive. Nonetheless, I want to explore the possibility that while Cantor's Theorem is of course true, it doesn't actually show that infinity comes in different sizes. Cantor's Theorem says that if A is a set, then there is no pairing (i.e., bijection) between the members of A and those of the power set PA. It follows that there are different (cardinal, but in an intuitive rather than mathematical sense) sizes of infinity given this Pairing Principle:

1. PP: Two sets A and B have the same size if and only if there is a pairing between them.

Given PP and Cantor's Theorem, if A is an infinite set, then A is a different size from PA. But of course PA is infinite if A is, so there are infinite sets of different size.

A number of people have disputed the sufficiency part of PP, since it gives rise to the counterintuitive consequence that the set of primes and the set of integers have the same size as you can pair them up. But you really shouldn't both complain that there are different infinities and that PP makes the primes and the integers have the same infinite size. I am going to leave the sufficiency of PP untouched, but suggest that the pairing condition might not be necessary for sameness of size, and I will offer an alternative. That alternative seems to leave open the possibility that all infinities are the same.

To think about this, start with this thought experiment. Imagine that there is a possible world w that has some but not all of the actual world's sets, but that it still has enough sets to satisfy the ZFC axioms just as (I shall suppose) the sets of the actual world do. The set membership relation in w will be the same as in the actual world in the sense that if A is a set that exists both w and the actual world, then A has exactly the same members in both worlds (and in particular, all the actual world members of A exist in w). Then here is something that might well happen. We have two sets A and B that exist both in the actual world and in w. In the actual world, there is a pairing f between A and B. A pairing is just a set of ordered pairs satisfying some additional constraints (the first element is always from A and the second is always from B, and each element of A occurs as the first element of exactly one pair, and each element of B occurs as the second element of exactly one pair). It might, then, be the case that although A and B exist in w, f does not--it exists in the actual world but not in the impoverished world w. It might even be the case that no pairing between A and B exists in the impoverished world. In that case, we have something very interesting: A and B satisfy the pairing condition in PP in the actual world but fail to satisfy it in w. If we are to satisfy the ZFC in w, this can only happen if both A and B are infinite.

Things might go even further. We might suppose that w only contains sets that are countable in the actual world. The mathematical (much less metaphysical!) possibility of such a scenario cannot be proved from ZFC if ZFC is consistent, but it follows from the Standard Model Hypothesis which a lot of set theorists find plausible. If w only contains sets that are actually countable, then any infinite sets in w will have a pairing in the actual world. There is, thus, an important sense in which from the broader point of view of the actual world, all infinite sets in w have the same size. But w is impoverished. There are pairings that exist in the actual world but don't exist in w, and so applying PP inside w will yield the conclusion that the infinite sets in w come in different sizes. However, intuitively, it still seems true to say that these sets in w are all the same size, but w just doesn't have enough pairings to see this.

Here's one way to argue for this interpretation of the hypothesis. Plausibly:

2. Pairing-Sufficiency: If there is a pairing between sets A and B, they are the same size.
3. Absolute-Size: If two sets are the same size in one possible world, they are the same size in any world in which they both exist.

Pairing-Sufficiency is one half of PP. Given Pairing-Sufficiency and Absolute-Size, if two sets have a pairing in any possible world, including the actual one, they have the same size in every world, including w. Thus, in w all the infinite sets in fact have the size, but you wouldn't know that if your tools were restricted to the pairings in w.

Thinking about the above scenario suggests a modification of PP to a Possible Pairing Principle:

4. PPP: Two sets A and B have the same size if and only if possibly there is a pairing between them.

Given that the members of a set cannot vary between possible worlds, I think PPP is at least as plausible as PP. Moreover, I think that if there are any cases (like my hypothesis that a world like w is possible) where PPP and PP come apart, we should side with PPP. Here's why. I think we go for PP as an abstraction from our general method of comparing the sizes of pluralities by pairing. (One imagines a pre-numerate people trading goats and spears in 1:1 ratio by lining up each goat with a spear.) But the natural abstraction from our general method is that if one could pair up the two sets, then and only then they are the same size. So PPP is the natural hypothesis. The only reason to go for PP is, I think, acceptance of PPP plus an additional hypothesis such as that what pairings there are doesn't vary between possible worlds.

If PPP (or just (2) and (3)) is true and my w hypothesis is a genuine metaphysical possibility, then it is metaphysically possible that all infinite sets are the same size--i.e., it could be that the actual world is relevantly like w. Furthermore, we clearly don't have relevant empirical evidence to the contrary. So, if all this works, it is epistemically possible that all infinite sets are of the same size. (Of course, the most controversial part of all this is the idea that what sets there are might differ--even in the case of pure sets--between worlds.)

But perhaps this won't satisfy the people who find size differences between infinities paradoxical. For they might find it paradoxical enough that there could be infinities of different sizes, something that was definitely a part of my story (remember that I started with two worlds, one in which there were differently sized infinities and an impoverished w with all infinities of the same size according to PPP).

I think I might be able to do something to satisfy them, while at the same avoiding the biggest problem with the above story, namely the assumption that what pure sets there are differs between worlds. Here's my trick. In the above, I assumed that pairings were all sets. But in line with the Platonism suffusing all of the above arguments, let's try something. Let's allow that there are pairings that aren't sets. Those pairings would be binary relations satisfying the right formal axioms. But here I mean "relations" in the Platonic philosopher's sense, not in the mathematician's sense where a relation is a set of ordered pairs. Let's suppose, further, that corresponding to any set of ordered pairs, there is a relation which relates all and only those pairs which are found in the set. In my earlier story, I made sense of the idea that two sets in w might not have a pairing in w and yet might be the same size by adverting to pairings that exist in another world (the actual one--and then at the end I flipped things around so that w was actual). But now we do the same thing by distinguishing between mathematical pairings--namely, sets of ordered pairs satisfying the right axioms--and metaphysical pairings--namely, Platonic binary relations satisfying analogous axioms. If my earlier story is coherent (I mean that to be a weaker condition than "metaphysically possible"), then so is this one: In w, there are infinite sets that do not have a mathematical pairing, but every pair of infinite sets possibly has a metaphysical pairing. But now this story doesn't rely on varying what pure sets exist between worlds. The story appears compatible with the idea that pure sets are the same in every world. But there are, nonetheless, metaphysical pairings that do not correspond to mathematical pairings, and PPP should be interpreted with respect to the metaphysical pairings, not just the mathematical ones. Note, too, that what metaphysical pairings hold between sets might differ between possible worlds, without any variation in sets. For some of the pairings may correspond to extrinsic relations. Here is an extrinsic relation that could turn out to be a metaphysical pairing, depending on what I actually was thinking yesterday: x is related to y if and only if x and y came up in one of my thoughts yesterday in this order.

We can now suppose that this story works in every possible world. Thus, assuming the coherence of the Standard Model Hypothesis, we have a mathematically coherent story--whether the metaphysics works is another question (the story is too Platonic for my taste, and I don't share the motivation anyway)--on which (a) all infinite sets are really of the same size (and hence of the same size as the natural numbers), (b) what pure sets there are does not differ between worlds, and (c) Cantor's Theorem and all the axioms of ZF or ZFC are true. If we were to go for such a view, we would want to distinguish between sets being of the size metaphysically speaking and their having the same mathematical cardinality. The latter relationship would be defined by a version of the PP with "pairings" restricted to the mathematical ones. And then mathematics could go on as usual.