In the movie A prisoner of Azkaban, Harry Potter is saved by a mysterious figure that looks like his father shortly before passing out. After that, he learns that time travel is real. He travels to the past and stops by the event to meet his father but no one comes to save him, so he decides to step up himself - only later realizing that he only saw himself.

Which philosophical theories that acknowledge the existence of free will are compatible with this scenario? In other words - let's imagine that the future is "determined" in the sense of "perfect predictability" but some features of the future are contingent on the influence of some cognitive structures I consider "the self". That is, free will, as conceptualized here, is part of what determines the universe. In this model of the world, can libertarian free will exist? Or is the mere predictability of what some free agent will freely do incompatible with libertarian free will?

  • Time travels are full of paradoxes which if associated with free will can seem as free will paradoxes when in reality they are time travel paradoxes. Keep this in mind.
    – Nikos M.
    Commented Apr 30 at 22:10
  • @NikosM. The presented model of time travel doesn't create any paradoxes
    – Probably
    Commented Apr 30 at 22:28
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    @Conifold the fact that under libertarianism one can freely choose to be predictable or freely follow some rules, is not a linguistic loophole. and certainly can clear the misunderstanding that libertarianism is completely incompatible with predictability
    – Nikos M.
    Commented May 1 at 7:54
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    'This kind of time travel is totally consistent with our understanding of physics.' Ha! Not with this physicist's understanding of physics. Commented May 1 at 10:52
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    @NikosM. The OP is talking about "perfect predictability", not "adequate amount of predictability", and libertarian freedom to do otherwise is not a perfectly "predictable process".
    – Conifold
    Commented May 1 at 17:39

3 Answers 3


Edited version, in honour of the charming Tkol...

I think you have answered your own question. If the future is entirely pre-determined, in the sense that there is no possibility whatsoever of it being other than it is bound to be, then any human decisions that have a bearing on the future also have to be entirely pre-determined. If you want to assume the sort of libertarian free will that allows you complete freedom of choice, then you have to abandon the idea of a pre-determined future.

As a certain commentator has pointed out (to preserve his or her anonymity I will simply refer to them as Tkol X) there are various ideas that try to combine the idea of moral agency with determinism, which come under the heading of compatibilism, but if you study them with the sort of unswerving rigour, supremely penetrative insight, and immense objectivity tinged with just the right amount of skepticism, as a certain poster does, then you will conclude that such ideas are really just an attempt to persuade you of a different meaning for 'free will', and are thus just changing the subject.

As for time travel of the sort you have in mind, it is a fantasy found in books and movies, so it is no guide to reality. You say in your comments that it is completely compatible with our understanding of physics- I beg to differ.

  • "If you want to assume free will then you have to abandon the idea of a pre-determined future. " What about compatibilism?
    – TKoL
    Commented May 1 at 6:50
  • That's irrelevant to my starting assumption about the future being predetermined. If it is then by definition future decisions are predetermined. Commented May 1 at 7:41
  • Yes, and even if they're determined, they could still be free if compatibilism is the case, right?
    – TKoL
    Commented May 1 at 9:30
  • No, you have changed the subject. Your compatibilist definition of free will is not the same as my definition of free will. I have made a point about apples and you have said 'but what about bananas?' Commented May 1 at 9:51
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    @TKoL in the meantime, I have edited my answer as a tribute! Commented May 1 at 11:08

Some conceptions of free will, compatibilist ones mainly (though perhaps not all compatibilist ones) see human minds as something like decision making machines. If the machine of your decision making apparatus outputs a choice given a particular environment, then, for all intents and purposes, that choice is "free" - and this remains true even if that decision making apparatus operates deterministically.

Thus, if Harry Potter goes back in time and does something, then even if we know that he was guaranteed to do that thing, it was still a valid and "effectively free" output of his decision making apparatus. It was still HIM that made that choice.


Harry Potter is a character in a fictional novel, and as such, does not have free will - every action he takes is controlled by the author.

This might sound a bit facetious or tongue-in-cheek, but I'm making a serious point too. This scenario from The Prisoner of Azkaban - i.e. this backward time-travel with the self fulfilling time loop (I've also heard it called the "Novikov Self-Consistency Principle") can only work when you have an author with tight control over the script.

This is so as to ensure that Harry neither has the motive, opportunity, nor enough information to change how the events transpired when he comes back around (and therefore attempt to demonstrate free will), and this can only be guaranteed in a work of fiction.

This question isn't about Harry Potter, of course. You could ask the same question about God.

Of course. But some argue that God, too, is a fictional character. And while others claim that he is real, they also accept that he occupies a set of laws outside of our mortal restrictions and "works in mysterious ways". Either way, he can't really be subject to the question in the same way that a normal person could.

  • There is the theory of time travel by having multiple universes, with one being exactly like ours a millisecond ago. So you don’t travel in time, you move to a different universe. And you don’t get a paradox, but you manage to travel into a universe that a minute from now will be very, very similar to ours now.
    – gnasher729
    Commented May 1 at 19:15
  • Fair point but I disagree. Imagine you see your future self doing something bad (punching a puppy). To prevent this future, you become a monk and train extreme morality and self-control. One day, you meet said puppy and become this future self. What happens at this moment? For some reason, you just really want to punch that puppy. You feel perfectly free to do otherwise, you just happen to want to act exactly the way you did. This is how this kind of time travel situations work if the physicists who think it can exist are right en.wikipedia.org/wiki/Novikov_self-consistency_principle
    – Probably
    Commented May 6 at 20:01
  • @Probably "What happens at this moment? For some reason, you just really want to punch that puppy. You feel perfectly free to do otherwise, you just happen to want to act exactly the way you did." - but how? How could time-travel be so inextricably linked with mind-control? This can only work in a scenario that is extremely rare and contrived (for which you an author with control of the universe). But if backward time travel were real it would have to cope with all sorts of random situations.
    – komodosp
    Commented May 7 at 13:45
  • To put it another way, it's quite possible to punch a puppy in spite of that training. (Which, BTW, you didn't know your future self had done - but what if you did?) But what if, say, you saw your future self fall to your death off a cliff because you weren't watching where you were walking. Now when you get to be the future person - knowing what's coming it's easily avoided. Are you saying you would just really want to walk off the cliff? And for some reason act all surprised as if it was an accident?
    – komodosp
    Commented May 7 at 13:56

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