Lewis and the Price of Time Travel: Lessons from _Großvater_

Responding to Lewis’s (1976) defense of the consistency of time travel (TT), Horwich (1987) and Price (1996) claim that TT may nevertheless be shown to be improbable, due to its need for unlikely coincidences. Smith (1997, 2024) and Ismael (2003) reply, correctly, that this begs the question against TT. Where does this leave us, and TT itself? To put the issue in a broader frame, I note (i) a Lewis-inspired “defense” of Aristotelian mechanics against a famous argument by Galileo; and (ii) the relevance of the Duhem-Quine Thesis. With a range of comparison cases thus in view, TT may be assessed by the Quinean pragmatic standards for theory choice that Lewis elsewhere endorses. I conclude that it is not unreasonable to conclude that TT is highly implausible, despite the fallacy of a direct appeal to the unlikelihood of coincidences. By Lewis’s own standards, treating Quinean pragmatic virtue as a guide to truth, this amounts to an argument for the epistemic improbability of TT.