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Wasserman presents a common argument against time travel:

(P1) If backward time travel were possible, it would be possible to perform a self-defeating act.

(P2) It is impossible to perform a self-defeating act.

(C) Backward time travel is impossible.

If one finds one example of a universe where backward time travel is possible and it is not possible to change the past, does this mean that time travel is possible?

(By time travel is possible in the universe, I mean there exist closed timelike curves in the universe (definition of time travel used by Smeenk and Wuthrich, 2011)

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    Thanks Mauro. Also, does finding one counterexample of a universe where backward time travel is possible and it is not possible to change the past, reflect that C is not true i.e. that backward time travel is NOT IMPOSSIBLE or does it show that backward time travel IS POSSIBLE.
    – Maths
    Commented Dec 11, 2018 at 9:41

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The above argument is valid.

Thus,

"if one finds one example of a universe where backward time travel is possible and it is not possible to change the past,"

means that the premsie (P1) is false, because the above statement is exactly the negation of (P1).

The fact that one premise is false does not invalidate the argument.

But, of course, the falsity of a premise implies that we are not licensed to assert that the conlusion is true.

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