Mary is a supporter of free will. Thanks to a time machine, she travels five minutes into the future and finds out that she will quietly drink a coffee. She goes back and decides not to drink it, to reject her discovery.

It follows that:

1) Once back in the present, something will force Mary in a few minutes to quietly drink a coffee. Even if this hypothesis is logically possible, it sounds not very persuasive.

2) Time travels are impossible.

3) In returning to the present, Mary find herself in a different time line. Consequently: a) there are several universes, b) in some cases it is impossible to visit the future of one's own.

4) Mary doesn't drink the coffee and changes the future. In this case, she had previously not visited her future, since she saw herself drinking a coffe, and this is not the case anymore.

is there a better answer, or it's just opinion-based?

This thought-experiment is inspired by a time-travel loop from Time and Space, by Barry Dainton:

The Marriage Decision Mary is torn between two suitors. She can’t decide whether to marry Tom or Jack. So she travels to the future and finds out that she is happily married to Tom. She then travels back, and marries Tom for this reason.

closed as primarily opinion-based by Swami Vishwananda, virmaior, Frank Hubeny, Eliran, Conifold Sep 30 '18 at 22:45

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    1) Would ve required by Novikov's principle. 3) Involves multiple universes. And there is no best answer as often in philosophy. – rus9384 Sep 29 '18 at 9:25
  • 2
    The best answer seems to be that time travel is not possible. The case against is surely overwhelming. – PeterJ Sep 29 '18 at 10:42
  • You omitted the possibility that freewill-believing Mary actually did alter the future by making a decision not to drink the coffee. Remember, with freewill the future is not determined. – Bread Sep 29 '18 at 13:56
  • @Bread You are right, but if she changes it, she should have seen another thing anyway (herself not drinking the coffee) – Francesco D'Isa Sep 29 '18 at 14:00
  • 1
    None of the answers follow, and I don't see any enigma here. The main issue here is whether the future is determined, and if you're interested in that question, you might want to ask it directly. (And if you're interested in time travel, there are paradoxes about changing the past that avoid the whole determined future and free will stuff.) – Eliran Sep 29 '18 at 16:20