The Paradoxes of Time Travel
Lewis (David)
Source: Lewis - Philosophical Papers Volume II, Part 4: Counterfactuals and Time, Chapter 18
Paper - Abstract

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  1. This paper argues that time travel1 is possible, and that the paradoxes of time travel2 are oddities, not impossibilities.
  2. The defence of the possibility of time travel3 involves
    1. a commitment to enduring things having temporal as well as spatial parts,
    2. psychological continuity4 and connectedness5 and
    3. causal continuity
    as criteria of personal identity, and a distinction between external and personal time.

Author’s Introduction
  1. Time travel6, I maintain, is possible. The paradoxes of time travel7 are oddities, not impossibilities. They prove only this much, which few would have doubted: that a possible world where time travel8 took place would be a most strange world, different in fundamental ways from the world we think is ours.
  2. I shall be concerned here with the sort of time travel9 that is recounted in science fiction. Not all science fiction writers are clear-headed, to be sure, and inconsistent time travel10 stories have often been written. But some writers have thought the problems through with great care, and their stories are perfectly consistent.
  3. If I can defend the consistency of some science fiction stories of time travel11, then I suppose parallel defenses might be given of some controversial physical hypotheses, such as the hypothesis that time is circular or the hypothesis that there are particles that travel faster than light. But I shall not explore these parallels here.
  4. What is time travel12? Inevitably, it involves a discrepancy between time and time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival (positive, or perhaps zero) is the duration of the journey. But if he is a time traveler13, the separation in time between departure and arrival does not equal the duration of his journey. He departs; he travels for an hour, let us say; then he arrives. The time he reaches is not the time one hour after his departure. It is later, if he has traveled toward the future; earlier, if he has traveled toward the past. If he has traveled far toward the past, it is earlier even than his departure. How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?


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