I want to consider how we know what worlds are metaphysically possible. First, let’s state what a possible world is. I like to think of possible worlds in the way Kripke states it: “‘Possible worlds’ are total ‘ways the world might have been’, or states or histories of the entire world (18).” So here’s the question: How do we know the ways the world might have been, or might be? Here I’m ignoring any abstract reality and restricting the question to causal reality.
One popular view on the epistemology of modality uses a principle that says conceivability is a guide to possibility—let’s call it CGP. We can see this principle in Hume’s Treatise:
‘Tis an established maxim in metaphysics, That whatever the mind clearly conceives includes the idea of possible existence, or in other words, that nothing we imagine is absolutely impossible. We can form the idea of a golden mountain, and from thence conclude that such a mountain may actually exist. We can form no idea of a mountain without a valley, and therefore regard it as impossible. (T 1.2.2)
The idea is that if we can conceive of something in some robust way, then that tells us that the world could have turned out that way. One counterexample to CGP is Goldbach’s conjecture: we can conceive of it being true or false, but it is necessarily one or the other. I think Chalmers’ more robust version of conceivability can bypass this objection. Another counterexample is Kripke’s a posteriori necessities involving natural kinds: e.g. we can conceive of water not being H₂O but their identity is necessary. For the purposes of this post, I’ll ignore Kripke since I think there are bigger problems with CGP. I’m concerned with the idea that by conceiving of a different coherent set of laws, we can know that the actual laws of nature could have been different.
The main problem with CGP is that there is no reason to think that a priori conceivability has anything to do with ways the world might have been. Granted things have to be logically possible for things to be metaphysically possible, but defenders of CGP are making a bolder claim. Suppose that we can coherently conceive of some laws of physics where things travel faster than the speed of light. How does that show us that the actual world could have turned out that way? Where’s the link? I suspect that defenders of the view take it as a brute intuition. A priori reasoning, while being indispensable in some areas, can only take us so far. Hegel went too far when he argued a priori that there cannot be more than six planets. Similarly, the CGP goes too far in my estimation.
Since I’ve ruled out a priori conceivability, and I’m not a skeptic about modal epistemology, the other option is that we can know what’s possible a posteriori. So how to we figure out our modal landscape? First, we know trivially that the actual world is possible, and we know what the actual world contains from experience. Second, if the laws of nature are indeterministic, that brings about additional possible ways that the world might have been. Of course, we find out about the laws of nature empirically. So the trick to figuring out “ways the world might have been” is to look at the actual world and figure out the laws of nature. If determinism is true, then there is only one possible world; if indeterminism is true, we have many possible worlds.
There’s an interesting result once you ground possibilities in the laws of nature in the actual world. It turns out that the fundamental laws of nature are necessary; they couldn’t have been different. Why? Because if possibilities come from the fundamental laws of nature (more specifically, the causal powers of actually existing things), and for the fundamental laws of nature to possibly be different, there would have to be even more fundamental laws to allow for that possibility, which, by definition, is impossible.
Graham Oppy makes an interesting observation about a view like this:
My favourite theory of modality has the evident advantage of theoretical frugality. On the one hand, if there are objective chances, then any theory of modality is surely committed to the possibility of the outcomes that lie in the relevant objective chance distributions. On the other hand, it is not clear that we have good reason to commit ourselves to any possibilities beyond those that are required by whatever objective chances there might be; at the very least, any expansion of the range of possibilities clearly requires some kind of justification. (47)
This theory has the advantage of theoretical frugality. Those who think that chances (or possibilities) can come somewhere besides the laws of nature owe us some kind of justification. If they think there are chances that do not come from any actually existing things, they are positing a sui generis chance—a chance from nowhere. Parsimony dictates that we not accept mysterious things without reason.
This theory also has the advantage of not falling to the “relevance problem.” If possibility is grounded in Plantingean propositions or Lewisian concrete worlds, what relevance do they have to how the actual world could have been? As William Lycan puts it:
…why should we suppose that real possibility and other modalities in this world have anything to do with specially configured sets of items, whether sentences or propositions or matter-elements? It seems unlikely that what fundamentally makes it true that there could have been talking donkeys is that there exists a fabulously complex set of some sort. (Lycan 1998, 92)
By contrast, if we ground possibility in the actual laws of nature, it has the advantage of being eminently relevant to how the actual world could have been.
Chalmers, David. “Does conceivability entail possibility?.” Conceivability and possibility (2002): 145-200.
Goldschmidt, Tyron, ed. The Puzzle of Existence: Why is There Something Rather Than Nothing?. Routledge, 2014.
Hume, David. A treatise of human nature. Courier Corporation, 2003.
Kripke, Saul A. “Naming and necessity.” Semantics of natural language. Springer, Dordrecht, 1972.
Lycan, William. 1998. “Possible Worlds and Possibilia:’ In Contemporary Readings in the Foundations of Metaphysics, Stephen Laurence and Cynthia Macdonald (eds). Oxford: Blackwell.