Wednesday, March 21, 2007

Intuitions and Metaphysical Possibility

In a recent paper, I offered my readers the following thought experiment:

"You are in the kitchen one day with your spouse and, suddenly, everything around you freezes. Your spouse stops moving, the clock stops ticking, the crickets stop chirping, but you continue to move normally. Suddenly, a man walks into the room. He explains to you that he is a time-traveler from the future. He takes you on several time-traveling trips with him and convinces you beyond any doubt that he has the power of time travel he claims to have. When you finally return to the kitchen, he informs you that when he leaves and time begins to move normally for everyone else again, he needs you to kill your spouse. If you do not, he explains, he will travel five years into the past and kill your spouse himself."

The purpose of this thought experiment was to probe my readers' intuitions on this story, with an eye towards making an argument about the unreliability of our intuitions when we are confronted with temporally abnormal situations.

After reading the paper, someone objected that I should not have used this thought experiment because it is not metaphysically possible. When I heard this, I was rather surprised, not because I disagreed about the possibility of this example (though I don't think it is metaphysically impossible as it stands; it is just metaphysically impossible on the most natural reading of it), but because I don't tend to think that it always matters whether or not thought experiments are metaphysically possible. Of course, if the goal of the thought experiment is, for instance, to find out what we should or should not do in a given situation, it is pretty silly to offer a situation that could never happen. But given that the end goal here was somewhat tangential to the intuitions themselves, the question of possibility never occurred to me.

Anyway, the point of this post is to find out what everyone else thinks. Should our thought experiments always be metaphysically possible? If not, is it always preferable that they be so? If not, should I still have used one in this particular instance? Inquiring minds want to know.


  1. I don't think that there should be a limitation of metaphysical possibility. An illustration like this one, or like unbreakable desert-cookies, or any number of other things really doesn't need to be possible in order to bring out the points that you want.

    If the purpose of the illustration is to draw attention to some general principle then I don't think that any illustration needs to be metaphysically possible.

    What makes time travel metaphysically impossible, anyway?

  2. Assuming that I have in mind the same notion of metaphysical possibility as my objector, the idea here is not that time travel is impossible, but that this particular example is because it involves altering the past.

    This is easy to explain if you've seen both the Back to the Future movies and the Bill and Ted movies. In the Back to the Future movies, McFly goes back in time and alters the past; the present he leaves from is different from the present he returns to at the end. It is argued that this sort of time travel story is metaphysically impossible, because it violates the Law of Non-Contradiction. The events that led McFly to travel back in time happened (because he traveled back in time) and they also did not happen (because he altered the past). This involves a contradiction, and therefore could not happen. The analogous part of my story is that (it is presumed) my wife has been around for the past five years and done lots of stuff. The image that most likely comes to mind is that if I do not kill her, she will just disappear, and the present will be altered. However, this would entail that she both did and did not die five years ago, again resulting in a contradiction.

    Think now of the Bill and Ted movies. In those, Bill and Ted time travel and they affect the past, but they do not alter it. Everything that happens happens because when they travel back in time they were (in a not very precise manner of speaking) already there and their actions had already led to them traveling through time in the first place. Thus, their actions are not necessarily metaphysically impossible.

    The reason I said that my thought experiment is not necessarily metaphysically impossible is that while the most natural reading is the one offered above, it is possible to tell the story--without contradicting what's written--without one of these temporal paradoxes. There are two ways of doing this, one of which I like and the other I don't.

    The Way I Don't Like:
    It is possible to appeal to a multiverse scenario in which the time-traveler goes back, and by killing my wife (who was not killed "last time") creates an alternate reality, into which he then transports me. The reason I don't like this is that it might affect our intuitions on killing her, because she doesn't ACTUALLY die in the real world, there just is a version of my wife who's dead in some alternate reality.

    The Way I Like:
    The contradiction in the story takes place if one assumes that the wife has been around the past five years no matter what I do. However, the time-traveler could add to his story by informing me that if I do not kill my wife, he will travel back in time five years, kill her himself, and replace her with a sophisticated, futuristic robot replica. This replica will be indistinguishable (to me) from my actual wife until the moment it deactivates (and it's faceplate flies open or something like that). Furthermore, this robot will deactivate itself as soon as I have made my decision (should that decision be that I will not kill my wife). Of course, the resulting situation is odd, because there is already a fact of the matter about whether or not "my wife" is a robot. But, it still appears that what that fact is depends on whether or not I decide to kill her. To some extent, it becomes just an epistemological problem; but I think the intuitive results are the same.

  3. One might think that intuitions are unreliable in any weird, extreme, or abnormal situation.

    It seems strange to me that one would hold that intuitions are reliable for any zany scenario so long as it is metaphysically possible (not that I'm charging anyone here with that view). Since it takes some figuring to determine what is and is not metaphysically impossible, it would be a surprise if this is what reliable intuitions tracked.

    Intuitions about a sufficiently unlikely (the mere physically impossible?) scenario should make us wary. I don't see that metaphysical impossibility gives our intuitions that extra push into unreliability.

  4. I think that the basic intuitions that are tested here involve a classic sort of trade off situation. This is the case regardless of the crazyness or possibility of the case.

    My own read of the time travel situation is that you would simply have 5 years of your current memory erased (because it never happened in your subjective frame) and replaced with a new set of memories. This isn't all that spooky because you would have actually lived those five years. You'd never know that you could have had 5 more years with your wife.

    On the other hand, if you kill her then you get to keep your great memories of 5 years of life with your wife, but you would have the painful memory of killing her in order to keep them.

    Your read of the situation is interesting, but I don't know why we would have to say that it violates non-contridiction. Why assume that there is some inviolable time line which you would have to branch off from? I've always pictured it more like splicing a film strip.

    The case is fairly simple in abstract. Either (a) you kill your wife and retain your memories or (b) you refuse to kill her, the time-traveler goes back and kills her 5 years ago and you have a substatially different life.

    It's an interesting question, but I don't know what context it was introduced in, so I can't really be sure that this is the point that you were trying to make.

  5. Mike,

    Briefly, the idea is this. Right now there is a fact about the past, namely that your wife lived from, say, 1995-2000. If you were to go back and kill her, the facts would be altered; your wife did not live from 1995-2000. This is problematic for two reasons. First, from the point of view of 2001, what are the facts? It seems that the facts are a contradiction: your wife both did and did not live from 1995-2000. This is the logical problem. The practical problem is this: If you went back and killed your wife so that she was not alive from 1995-2000, then how could it be that the time traveler came back and threatened you? Your wife would already have been dead.

    I think the thing that makes this confusing is that it's easy to think of it like this: The FIRST time around, your wife was alive and then the guy went back and killed her so that the NEXT time around she was already dead. But this isn't how time works; each time occurs only once.

    This was sort of haphazard; so just let me know if any of it is (still) confusing.

  6. I think I'm with Arthur on this one. The further one deviates from some recognizable version of reality, the more unreliable I would expect our intuitions to be. Of course, the more fantastic the scenario, the more interested I am in reading it, but that's because I enjoy reading about fantastic scenarios anyway, and having a discussion about intuitions just becomes an entertaining meta-fiction.

    That said, the most common problem I've seen is that people just don't share the same metaphysical intuitions.