Monday, April 25, 2005

Real Possibilities

Do most philosophers treat possible worlds as "real" ontological entities? That seems a bit extravagant to me. But here's the dilemma. If we take possible worlds as mere constructions, or "useful fictions", as I'm inclined to do, then we cannot use them to ground metaphysical possibility. For in constructing these "worlds", we must choose and impose a framework of contraints (e.g. the laws of classical logic, or of physics) regarding what we will allow in them. But that makes modality inherently relational: P is possible or not according to framework F; but there is no 'absolute' or transcendant framework which allows us to say that P is possible, simpliciter.

So suppose instead that there is such a transcendent framework, as provided by the set of "objectively real possible worlds". We can then say that P is metaphysically possible (i.e. possible simpliciter) iff P is true in some ORPW.

But, given that all we have epistemic access to is the actual world, how could we know anything about these possible worlds or their contents? How are we supposed to know what set of constraints they are governed by? How could we even guess? What's the basis for this wild metaphysical theorising?

Further, what does it even mean to say that something is metaphysically "possible", or that "the world could have been that way"? I can't make any sense of it. There's the way things are, actually, and then there's the way things are in other possible worlds, but what's the connection?

In what sense are other possible worlds supposed to be really possible? What does it mean to say, for example, "the universe could have been such that Kerry got elected instead"? Are we saying that our world could have been a different possible world? Isn't this utterly nonsensical?

Or perhaps it isn't a claim about the identity of these worlds, but rather, about which of them got actualized. The claim might be understood as: "Our world happens to be actual. But it need not have been. Any other possible world might have been actualized instead -- e.g. the one where Kerry became president." I'm not sure this makes any more sense though. What does that "might" mean, after all? Presumably it's a claim about metaphysical possibility -- so does that mean there is another possible world wherein the Kerry-world is actualized?

I'm so very confused. Please help!


  1. Whether you take possible worlds to be concrete (like Lewis) or abstract (like pretty much everyone else, Plantinga specifically comes to mind), you're presumably going to have them limited in the same ways. Also, notice that for people like Plantinga this 'possible worlds' talk is just a convenient analogy for 'ways things might have been.' The word 'might' is merely a modal term, nothing more. There are various ways we can restrict it--epistemically, metaphysically, nomologically, etc--but the main idea is that the world might have had countless differences. To say that a world is metaphysically possible is to say that our world might have turned out that way. That is, there are no metaphysical laws that make it such that everything in our world had to have been just this way. Of course, there are some metaphysical laws that say our world couldn't have turned out certain other ways. For instance, our world couldn't have had one plus one equal three. That is a metaphysical impossibility, or so those fond of modality would say.

    Lewis thinks that 'actuality' is an indexical term. He defines possible worlds as 'spatio-temporally isolated concrete individuals'. Our world is actual for us but some other possible world is actual for those people at that possible world. I hope at least some of this makes the notion a bit clearer.

  2. Hmm, I think my problem is with people making claims about "metaphysical laws". I'm not sure what could justify making any such claims. Everyone assumes that logical impossibilities are also metaphysically impossible. But how could we know that?

    (I talk a bit more about the problems with taking logic or conceivability as grounding metaphysical modality in the linked-to post.)

    I just don't see how we have any grasp whatsoever on how the world "might have been". I'm happy to talk about what's logically possible, or nomologically possible, or whatever, but not any transcendent "metaphysical possibility", since it doesn't seem to come with any clearly defined constraints (or indeed any constraints whatsoever).

    As for my other problem: consider modal realism, with its spatio-temporally isolated universes. How are they modally related? In what sense does it make sense to say that our world "could have been" different, i.e. could have been one of the other worlds?

  3. According to our best understanding of quantum mechanics, all physically possible worlds exist. As it is physically possible for me to have typed a "B" at the beginning of this comment, there are worlds where I did so. The phenomenom of quantum interference provides support for many worlds. See David Deutsch's Fabric of Reality or the Fabric of Reality list for more information (
    At a more speculative level, some believe that all logical possible worlds exist. The set of all logical possible worlds (called the Plenitude) contains zero information and is thus a much simpler entity than any particular world. For more information, see the everything list ( or Max Tegmark's website (

  4. Richard,

    You seem to agree with Quine about many of these things. Also, very quickly, people typically use 'logical necessity' and 'metaphysical necessity' synonymously. So if you're happy to talk about logical necessity and possibility, many would say you're ready to talk about modality in the way it is typically used.

  5. First, in order to have a reasonably clear discussion of whether or not "metaphysical possibility" is a clear or meaningful term, we need some sort of meaningful definition of "metaphysical", which is lacking. I can't think of such a definition that would (a) be reasonably consistent with the actual usage of the term, and (b) make "metaphysical possibility" anything distinct from "logical possibility".
    Second, I strongly object to the notion of "useful fictions" as anything distinct from "real things", as long as their utility consists in the role they play in our most accurate scientific theories. As Brian pointed out, possibility figures strongly in one interpretation of quantum mechanics, so we kind of have to say that it exists. But there are two objections we could raise to that: (1)interpretations of quantum mechanics come and go, so who knows how long possibility is going to play so important a role; (2) we are only obligated to accept judgments about possibilities in the senses in which they occur in our useful theories. I suspect that not all of our uses of the expressions "might have been", "possibly", etc. are synonymous with the uses of the notion of possibility in these theories, and so we might be able to scrap them. But actually confirming this hunch would take some work.

  6. Ian - The Many Worlds Intepretation of Quantum Mechanics has been on the ascendency for some time now. It assumes nothing more than the unitary evolution of the Schrodinger wave-equation and derives from this that other worlds must exist. Remarkably, as David Deutsch has shown, Many Worlds can be used to derive the Born probability rule of Quantum Mechanics. Other interpretations must assume this. Other interpretations must also add ancillary postulates, such as the non-local collapse postulate of the Copenhagen Interpretation. Saying that interpretations come and go is no more useful than saying theories come and go. One interpretation - Many Worlds - has withstood all attempts at critical refutation while requiring fewer assumptions and must be taken seriously for this reason. It is not in fact an interpretation. It is quantum mechanics.

  7. Chris, to be honest I know next to nothing about the literature on this topic. It just seems to me that the concept of "metaphysical possibility" is incoherent. It would be reassuring if Quine has made similar suggestions, since I had thought I must just be misunderstanding something. And perhaps I still am. Do you know of any responses to the sorts of objections I've raised? Could you recommend any books or articles that get at these issues?

    Chris & Ian, I think logical possibility does not mean the same thing as metaphysical possibility. The latter I understand as concerning how the universe really might have been. This is a claim about the nature of reality. Logical possibility, by contrast, is simply about what is allowed according to the rules of classical logic. It makes claims about a formal system (i.e. a human construction, or "useful fiction"), not about reality.

    Now, perhaps philosophers have typically taken logical possibility as the criterion of what is metaphysically possible. That is, they assume that classical logic provides the transcendent constraints on what is really possible. But they're still two distinct concepts. (And I think that only one of them actually makes the slightest bit of sense!)

    Ian, science can't help here, since it can only tell us what's nomologically possible (i.e. possible according to the laws of nature/physics). Metaphysical possibility is usually supposed to be broader than this, e.g. it could have been that objects could travel faster than light (if the laws of physics could have been different).

  8. I was under the impression that the metaphpysical and logical possibility are taken as being synonymous because they have identical truth conditions:

    P is logically possible iff it is true in some possible world.

    P is metaphysically possible iff it is true in some possible world.

  9. I think it is a bit dangerous to say "quantum physics cant help us" particularly whee quantum physics dspues one of your key asumptions.

    e.g. "But, given that all we have epistemic access to is the actual world,"

    I sugest cross polination of fields is quite valuable.

    unles you are trying to add "if quantum physics did not exist" to one of your metaphysical probibilities but I would suggest none of us have anywhere near the brain power to make even the most vaguely correct analysis of that potential world unless we bury it in assumptions.

    It reminds me of the old exam question "what if there was no friction what would happen to this car?"

    With interesting answers like
    1) it would start but the wheels would spin and the car wouldn't move
    2) it wouldnt go
    3) it would fall apart
    4) you would die instantaniously so who cares?

    I suggest the answers get closer to the "nearest posible world" as you go down.

  10. Someone said: "Logical possibility, by contrast, is simply about what is allowed according to the rules of classical logic. It makes claims about a formal system (i.e. a human construction, or "useful fiction"), not about reality."

    I shan't engage on the basic point about metaphysical possibility being about reality where logical possibility (LP) is a matter of our logic, but I think this is not quite correct that LP-claims are about "formal systems". Classical logic will rule out some logically impossible propositions as contradictions - those whose contradictoriness derives from the use of propositional connectives like "or", "and" and "not" and quantifiers like "some" and "all". There are debates as to whether it even does this job correctly, particularly with the "if .. then .." construction.

    There are other things and states of affairs which are logically impossible, such as married bachelors and changing the past, but we say these are so because that's how natural, non-formal language works.


Visitors: check my comments policy first.
Non-Blogger users: If the comment form isn't working for you, email me your comment and I can post it on your behalf. (If your comment is too long, first try breaking it into two parts.)