The philosophy of time

Questions to think about for 01.06.2016 (Week 7)

Vranas: Can I kill my younger self? Time travel and the retrosuicide paradox

  1. What is the retrosuicide paradox, intuitively put?

    If backwards time travel is possible, then it seems that it should be possible that one travel back in time and kill oneself. But that seems impossible, for then would not have gotten to the point of being able to travel back in time to kill oneself.

  2. What does Vranas claim is the hidden assumption in the standard (i.e. Lewis's) solution to the retrosuicide paradox?

    The assumption is that retrosuicide is possible, hence compossible with certain facts. However, if it is impossible, then it is compossible with nothing, and so the standard solution fails.

  3. What are three ways Vranas mentions that would make the hidden assumption harmless?

    The assumption would be harmless if (i) it were obviously true and so didn't need argument, (ii) it wasn't even needed in the sense that the solution could be reformulated without reliance on the assumption, (iii) it is required so that there is paradox at all.

  4. What is Vranas's objection to the three ways mentioned above that would save the standard solution from the "hidden assumption objection"?

    1. Is it obviously true? Suppose I am in a retrosuicide-propitious situation and the boy in front of me is myself. Then even if it is possible for me to kill the boy in front of me, in the sense that the sentence 'It is possible that Michael De kill the boy in front of him' is true because, in another world, 'the boy in front of me' can refer to a boy that is not an earlier stage of myself, the sentence 'It is possible that Michael De kill an earlier stage of himself' is not. In another possible world in which the boy in front of me is e.g. a mere duplicate of an earlier stage of myself, the sentence 'It is possible that Michael De kill the boy in front of him' does not even pick out a retrosuicide-propitious situation.
    2. Is it dispensable? One way to maintain that the hidden assumption is not necessary is to claim that whether someone has an ability depends on relevant facts and that this need not be spelled out in terms of compossibility. Thus I can have the ability to commit retrosuicide even though doing so is impossible and hence not compossible with any relevant facts. The problem, claims Vranas, is that independently of how one spells out ability, the following fact is plausibly true: (P5) Necessarily, if it is impossible for me to kill my younger self, then relative to the relevant features of any situation I cannot kill him. Spelling out ability in terms of compossbility is one way to get P5, but not the only one, and Vranas claims that any plausible such spelling out will entail P5.
    3. Is it innocuous? If retrosuicide is impossible (i.e. the hidden assumption is false), then it seems there's no paradox left. For there to be paradox we need a set of propositions all intuitively plausible but jointly incompatible. If the hidden assumption isn't intuitively plausible, no paradox. Vranas has three things to say here. First, the truth of the assumption is independent of whether one is justified in believing it, so one may still be justified in thinking there's paradox where there really is none. Second, even if one is justified in rejecting the assumption, there may still remain a paradox until one explains why the assumption is intuitively plausible. Why does it seem that I can commit retrosuicide when I can't? Third, the assumptions bears on whether one can commit retrosuicide, so it is not "innocuous" in that sense.

  5. Part of the appeal of the standard solution is that it respects these two conflicting intuitions we have about retrosuicide, namely, that it is both possible and not. Once we see that retrosuicide is impossible, how does Vranas propose we explain the lingering intuition that it's possible?

    It seems that we can commit retrosuicide because we can be in a qualitatively identical situation and kill the person that is a qualitative duplicate of our younger selves. It also seems that ability "supervenes on local physical features" so that if one can kill a duplicate of their younger self, it seems that they can commit retorsuicide. The problem is that this supervenience principle obviously but counterintuitively fails.