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Freedom, self-prediction, and the possibility of time travel

Abstract

Do time travellers retain their normal freedom and abilities when they travel back in time? Lewis, Horwich and Sider argue that they do. Time-travelling Tim can kill his young grandfather, his younger self, or whomever else he pleases—and so, it seems can reasonably deliberate about whether to do these things. He might not succeed. But he is still just as free as a non-time traveller. I’ll disagree. The freedom of time travellers is limited by a rational constraint. Tim can’t reasonably deliberate on killing his grandfather, certain that he’ll fail. If Tim follows his evidence, and appropriately self-predicts, he will be certain he won’t kill his grandfather. So if Tim is both evidentially and deliberatively rational, he can’t deliberate on killing his grandfather. This result has consequences. Firstly, it shows how evidential limits in the actual world contribute to our conception of the future as open. Secondly, it undercuts arguments against the possibility of time travel. Thirdly, it affects how we evaluate counterfactuals in time travel worlds, as well as our own. I’ll use the constraint to motivate an evidential and temporally neutral method of evaluating counterfactuals that holds fixed what a relevant deliberating agent has evidence of, independently of her decision. Using this method, an agent’s local abilities may be affected by what happens globally at other times, including the future.

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Notes

  1. While Lewis (1976, p. 150) takes the evaluation of abilities to depend on context, he implies that the only contexts in which Tim’s failure are held fixed are fatalist ones, and don’t concern what Tim can do in any reasonable sense.

  2. Even if spacetimes involving closed timelike curves allow for no global time ordering, so that we can only define temporal direction locally, and backwards time travel doesn’t involve local backwards causation, we still face problems evaluating counterfactuals that aren’t local—see Sect. 6.

  3. I won’t distinguish sharply between actions and their intended results. For some purposes, a more general formulation is preferable, one that uses de-tensed propositions or states of affairs, so as not to presume agents deliberate only on the future. But, for grammatical ease, I’ll use the future-tensed action formulation.

  4. For related notions, see Levi (1980, 1986), Harman (1976, p. 438) and Dennett (1984, p. 113).

  5. Admittedly, it’s difficult to make sense of Tim’s motivation for travelling back without taking him to be irrational, or to have adopted a non-standard metaphysics of time. See Objections 5 and 6 below.

  6. What if Tim allows for the possibility of young-gramps’ regeneration? Plausibly Tim intends to kill young-gramps in a permanent manner. For simplicity, I’ll take killing to imply permanently-killing. In any case, Tim aims to do something that will make the future different from what he believes it was.

  7. I take causal relata to be states of affairs, although, for this paper, nothing significant hangs on this choice; I’ll also talk loosely of other relata.

  8. There are other senses of trying which Tim would be unreasonable to engage in (see Objection 8).

  9. Such agents might continue to take themselves to act, even when they no longer deliberate, or might take themselves to become mere ‘instruments of the world’. Smith (2005) explores other attitudes the time traveller might adopt.

  10. A similar result can be reached by requiring deliberation to be between incompatible alternatives: certainty of intended result rules out other incompatible options as available.

  11. If the causal relata are fine-grained, time travel without causal loops may not be possible. Even so, the time traveller’s freedom would not be affected in virtue of the potentially self-defeating nature of the casual loop, but in virtue of its evidential features.

  12. Generalising moves of a similar form are made by Prosser (2016, Chap. 7) and Ismael (2007, 2011).

  13. For responses to objections involving curiosity, see Smith (1997, pp. 383–385).

  14. More carefully, B can’t be known in a way that would leave the correlation between A and B intact.

  15. One might still argue that an agent can bilk the correlation, even if she doesn’t. But then one would need to hear more about why this sense of ‘can’ is relevant to the correlation being causal, rather than one more closely tied to what an agent can reasonably deliberate on.

  16. If counterfactuals are context-sensitive, we may need to specify that these are the kinds of counterfactuals relevant for decision-making and abilities.

  17. Arguably, this is a necessary but not sufficient condition, since a causal condition may be required as well. My arguments won’t depend on this detail, or others concerning the condition’s content.

  18. Perhaps ignoring a ‘transition period’ just prior to the antecedent (Lewis 1979).

  19. For a related argument, see Kiourti (2008, pp. 349–350). Kiourti argues that future events are relevant to the time traveller’s ability to kill her younger-self because they’re in her ‘personal’ past. The argument I give here is more general: it does not rely on demarcating the time-traveller’s personal past, or on facts concerning the identity of the time traveller.

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Acknowledgements

I would like to thank the following people for helpful comments, discussions and suggestions: David Albert, Achille Varzi, Nina Emery, Daniel Nolan, David Braddon-Mitchell, Mike Hicks, Zee Perry, Helen Beebbee, Robbie Williams, Tom Dougherty, Tim Button, Lukas Skiba, Nick Gorman, and audiences at the Joint Session, the Pacific APA, and the International Association for the Philosophy of Time Conference. This work was supported by a Postdoctoral Fellowship at the Center for Philosophy of Science, University of Pittsburgh and a Research Fellowship at the University of Warwick on an Arts and Humanities Research Council project ‘Time: Between Metaphysics and Psychology’ (AH/P00217X/1).

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Fernandes, A. Freedom, self-prediction, and the possibility of time travel. Philos Stud 177, 89–108 (2020). https://doi.org/10.1007/s11098-018-1181-9

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Keywords

  • Time travel
  • Freedom
  • Deliberation
  • Causation
  • Counterfactuals
  • Time asymmetry