In “The Paradoxes of Time Travel” (1976), the late philosopher David Lewis developed an intriguing line of argument to the conclusion that time travel is possible, though it certainly does not seem so in our everyday experience. Here we seem to face a paradox: on the one hand, it seems that time travel is possible (in a possible world as characterized by Lewis); on the other hand, it seems that time travel is not possible (in our actual world). What shall we make of this puzzle, a seeming paradox?
Part A.
To begin, suppose with Lewis that time travel is possible: how then would it work? Lewis’ argument assumes a distinction between “external time” and “personal time”. Explain this distinction: how is “external” time characterized, and how by contrast is “personal” time characterized, in Lewis’ account? Apply this distinction to a concrete example: a fictional case of time travel, where you travel through time and there is a difference between external time and your “personal” time during your time travel. Lewis writes, “We may liken intervals of external time to distances as the crow flies, and intervals of personal time to distances along a winding path. The time traveler’s life is like a mountain railway. … ” (2nd page, bottom of 2nd column). Explain how this analogy applies to the distinction as drawn in your fictional case of time travel.
Part B.
Lewis’ account of possible time travel assumes a basic metaphysical model of the world, according to which: “The world — the time traveler’s world, or ours — is a four- dimensional manifold of events. … Enduring things are timelike streaks: wholes composed of temporal parts, or stages, located at various times and places.” (1st page, 2nd column.) Suppose you are a time traveler, an “enduring thing”, as in the fictional case described in Part A. Explain Lewis’ model of you during your time travel: you have “temporal parts”, or “stages”, in which different events happen. What happens in these different stages of “you”? How is their location in time specified, i.e., in external time or in personal time or both? How does the story of you and your journey unfold in time?
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