Desert, free will, determinism

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Re: Desert, free will, determinism

Postby davidm on September 16th, 2018, 2:54 pm 

If a person goes back to the past, he has not changed anything. He was always in the past, and anything he did there was always true.
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Re: Desert, free will, determinism

Postby davidm on September 16th, 2018, 2:58 pm 

ronjanec » September 16th, 2018, 12:32 pm wrote:
Say the same person goes back to the past and steps on a Black Widow spider and then kills it; If you never went back....


But he did go back. Ergo it's always (timelessly) true that he went back. It's not possible that he did not go back. If he did not go back, it's not possible that he did go back.
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Re: Desert, free will, determinism

Postby ronjanec on September 16th, 2018, 3:28 pm 

davidm » Sun Sep 16, 2018 12:54 pm wrote:If a person goes back to the past, he has not changed anything. He was always in the past, and anything he did there was always true.


A person goes back to the past, after previously existing in what is called the present; How could: “He was always in the past,” also be true?(One statement contradicts the other davidm)
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Re: Desert, free will, determinism

Postby davidm on September 16th, 2018, 3:49 pm 

There is no contradiction in what I have said. You are committing the classic "second-time around fallacy."

If someone travels to the past, he was already in the past. Before someone travels to the past and kills a black widow spider, he had already killed said spider, in the past.
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Re: Desert, free will, determinism

Postby davidm on September 16th, 2018, 4:04 pm 

A man is born in 2000. In 2018, he steps into a time machine, and travels back to 1999, the year before he was born. In 1999, he steps on a black widow spider.

This means he stepped on the spider before he was born.

Nothing about the past has changed. It is timelessly true that he stepped on the spider.

If, in 2018, he decides NOT to step into the time machine and go the past, then before he was born, the spider was NOT stepped on, and "spider NOT stepped on" is timelessly true instead.

So-called time travel paradoxes have an identical logical structure to the so-called problem of omniscient foreknowledge cancelling free will. There are no time travel paradoxes, and omniscience does not negate free will, and for the same logical reason.
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Re: Desert, free will, determinism

Postby davidm on September 16th, 2018, 4:37 pm 

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Re: Desert, free will, determinism

Postby ronjanec on September 16th, 2018, 7:18 pm 

davidm,

I’m still not getting this...

But despite this, I personally have a very important reason(s) why time travel will always be impossible no matter what time travel scenario anyone else comes up with, and I will again try to start a new thread on this in the coming week(it is in regards to the fact that the time traveler’s theoretical ”destination” does not literally exist anywhere in objective realty)

(Hopefully you will also participate in my new thread davidm)
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Re: Desert, free will, determinism

Postby DragonFly on September 16th, 2018, 8:15 pm 

It seems that no one from the future has arrived here demonstrating technology greatly beyond our own.
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Re: Desert, free will, determinism

Postby ronjanec on September 16th, 2018, 8:40 pm 

DragonFly » Sun Sep 16, 2018 6:15 pm wrote:It seems that no one from the future has arrived here demonstrating technology greatly beyond our own.



And no one ever will DF; What is only called the ‘future’ today by man, does not literally physically exist anywhere(or have distinct physical objective existence anywhere), and neither do the people who will be alive in what is today called the much distant future, possibly with a technology greatly beyond our own.
Last edited by ronjanec on September 16th, 2018, 9:21 pm, edited 1 time in total.
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Re: Desert, free will, determinism

Postby Positor on September 16th, 2018, 9:20 pm 

DragonFly » September 16th, 2018, 4:40 pm wrote:
Positor » September 15th, 2018, 10:11 pm wrote:Wouldn't such indeterminacy almost certainly produce 'junk' rather than 'information' …

Most likely, "almost".

Doyle says, "The Cogito model simply identifies the source of randomness as the inevitable noise, both thermal noise and quantum noise, that affects both proper storage of information and accurate retrieval of that information at later times. These read/write errors are an appropriately random source of unpredictable new ideas and thoughts that provide alternative possibilities for action."

If randomness/noise plays a significant role in our thoughts, and most (even if not all) of it is junk, then it seems to me that we would experience the junk as such. That is to say, our mental experience would be full of perplexing, dreamlike discontinuities. But we do not (normally) experience such discontinuities; events seem to proceed from one moment to the next in a very precise way. So I think any randomness must be negligible at the level of our thought processes, and therefore too small to be relevant to the question of free will.
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Re: Desert, free will, determinism

Postby Positor on September 16th, 2018, 9:56 pm 

davidm » September 16th, 2018, 9:04 pm wrote:A man is born in 2000. In 2018, he steps into a time machine, and travels back to 1999, the year before he was born. In 1999, he steps on a black widow spider.

This means he stepped on the spider before he was born.

Then he would have to have existed before he began to exist, which is a contradiction.

To look at it another way: In 1999, he steps on the spider. The 'he' who steps on the spider remains alive in 2000. Then 'he' is born in 2000 (this is timelessly true). So then there must be (and always have been) two of 'him' in existence at the same time.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 1:12 am 

Positor » September 16th, 2018, 7:56 pm wrote:Then he would have to have existed before he began to exist, which is a contradiction.

To look at it another way: In 1999, he steps on the spider. The 'he' who steps on the spider remains alive in 2000. Then 'he' is born in 2000 (this is timelessly true). So then there must be (and always have been) two of 'him' in existence at the same time.


It is not a contradiction. He, and everyone, has temporal parts, just as we all have spatial parts. His temporal parts would constitute a world tube that bends backward in the time dimension, while everyone else's world tubes are extended toward the future in spacetime. The temporal part that exists before he was born lies in his own subjective future, while it lies in the objective past of everyone else.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 1:23 am 

DragonFly » September 16th, 2018, 6:15 pm wrote:It seems that no one from the future has arrived here demonstrating technology greatly beyond our own.


It is theoretically possible to build a time machine. Paul Davies wrote a book about this. However, it comes with an important catch: You will not be able to travel backward in time, to a point prior to when the first machine was activated. Since no such machine has yet been built, we necessarily do not encounter time travelers.
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Re: Desert, free will, determinism

Postby DragonFly on September 17th, 2018, 2:02 am 

THE UNDETERMINED

The Alef as indeterminism
Marks LaPlace’s Daemon as unrisen,
And banishes Necessity’s prison—
The Universe is wholly contingent.

Randomness’s information content
At Alpha is zero, of purpose rent,
For maximum entropy fills the tent,
Devoid of any design of intent.

That the ‘verse expands is the saving grace,
For the difference between the swelling pace
As the max possible entropy’s race
And its base is potential info’s trace.

The first particle chisels the cooled stone,
The bit of info that begins the poem.
Gravity sends some into a star home;
Death throes spew atomic elements sown.

Yes, the Dark Reign of entropy will win,
But not until Omega’s closing yin.
In rare redoubts, info adds to its bin,
And life’s purpose to survive doth begin.

Embodied in the brain’s material,
Information signals through energy;
It's physical but immaterial,
Realizing ultimate activity.
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Re: Desert, free will, determinism

Postby Positor on September 17th, 2018, 8:07 am 

davidm » September 17th, 2018, 6:12 am wrote:
Positor » September 16th, 2018, 7:56 pm wrote:Then he would have to have existed before he began to exist, which is a contradiction.
To look at it another way: In 1999, he steps on the spider. The 'he' who steps on the spider remains alive in 2000. Then 'he' is born in 2000 (this is timelessly true). So then there must be (and always have been) two of 'him' in existence at the same time.

It is not a contradiction. He, and everyone, has temporal parts, just as we all have spatial parts. His temporal parts would constitute a world tube that bends backward in the time dimension, while everyone else's world tubes are extended toward the future in spacetime. The temporal part that exists before he was born lies in his own subjective future, while it lies in the objective past of everyone else.

OK, that seems clear enough. A 'theory of everything' would need to include all his temporal parts, and the relationship between the subjective and objective aspects. (He is part of the universe, so his subjective history is part of the universe's history.) But I suppose this could be done in principle. However:

1. Consider the 'objective' view (the point of view of everyone else). If he kills the spider in 1999, then he always did so. So other people must have noticed that he did so, and that he therefore existed in 1999. But these same people must have also noticed that 'he' was born in 2000. If he explains to them that the person born in 2000 is actually himself (and assuming that they believe him), then everyone (including him) must agree that there are two manifestations of him (two 'temporal parts' of him, if you like) co-existing in the same frame of reference. And he can interact with his other temporal part. Correct?

2. It seems that we can rule out time travel to any spatiotemporal location for which there is objective evidence that he was not present. (I think this is agreed.)

3. I wonder if there might be problems with time travel with regard to the conservation of energy. It would certainly add an extra level of complexity to the mathematical formulations.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 10:01 am 

Positor » September 17th, 2018, 6:07 am wrote:
davidm » September 17th, 2018, 6:12 am wrote:
Positor » September 16th, 2018, 7:56 pm wrote:Then he would have to have existed before he began to exist, which is a contradiction.
To look at it another way: In 1999, he steps on the spider. The 'he' who steps on the spider remains alive in 2000. Then 'he' is born in 2000 (this is timelessly true). So then there must be (and always have been) two of 'him' in existence at the same time.

It is not a contradiction. He, and everyone, has temporal parts, just as we all have spatial parts. His temporal parts would constitute a world tube that bends backward in the time dimension, while everyone else's world tubes are extended toward the future in spacetime. The temporal part that exists before he was born lies in his own subjective future, while it lies in the objective past of everyone else.

OK, that seems clear enough. A 'theory of everything' would need to include all his temporal parts, and the relationship between the subjective and objective aspects. (He is part of the universe, so his subjective history is part of the universe's history.) But I suppose this could be done in principle. However:

1. Consider the 'objective' view (the point of view of everyone else). If he kills the spider in 1999, then he always did so. So other people must have noticed that he did so, and that he therefore existed in 1999. But these same people must have also noticed that 'he' was born in 2000. If he explains to them that the person born in 2000 is actually himself (and assuming that they believe him), then everyone (including him) must agree that there are two manifestations of him (two 'temporal parts' of him, if you like) co-existing in the same frame of reference. And he can interact with his other temporal part. Correct?

2. It seems that we can rule out time travel to any spatiotemporal location for which there is objective evidence that he was not present. (I think this is agreed.)

3. I wonder if there might be problems with time travel with regard to the conservation of energy. It would certainly add an extra level of complexity to the mathematical formulations.


(1) is correct. As to (2), I’m not sure you could get such objective evidence. A time traveler could disguise himself. Per (3), I don’t see any conservation problem. We can use a spatial metaphor and imagine a snake curling about back on itself so that its head meets its own tail. The snake may not realize it (snakes have tried eating their own tails, and there is even a symbol for this, the ouroboros), but the head and tail are not different entities but two parts of the same entity, the whole snake. Likewise the version of the time traveler in 1999 and the version born in 2000 are two temporal parts of a unitary whole, a world tube.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 10:08 am 

If you're thinking of a conservation problem in terms of the time traveler suddenly appearing in the past, whereas before he was not there, this again would be the second-time-around fallacy. There is only one past, and if the traveler is there, he was always there.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 10:26 am 

I don't think the time travel sideline is a diversion from the main topic, because I think there is a deep logical connection between time travel scenarios, and determinism/free will scenarios. To that end I'm not really interested in whether time travel to the past is physically possible (though that is an interesting topic, and may require its own thread). It may or may not be possible. I'm only interested in showing backward time travel is logically possible, which I think I have already done, and examining the implications of this, and how those implications relate to determinism/free will.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 10:47 am 

Briefly, the connection is this: Suppose I enter a time machine with the goal of doing x. But even before I enter the time machine, I was already in the past, and in fact there I did y, and not x. If I now go back in time and try to do x, I must fail, because I had already done y.

Now the free will objection arises: If I am not free to do x, I have no free will.

But to do x when in fact I already in the past did y would violate the law of noncontradiction. And no valid account of free will entails violating the laws of logic. So this can’t be used as an argument against free will.

What this scenario shows is that no one, on pain of logical contradiction, can go to the past and also change it. But this is no different from ordinary future-directed time travel. I can’t change the present or future, either. What I can do, by freely acting, is make the past, present and future be what they were, are, and will be. This is not the same thing as changing the past, present or future.

If I can’t change the future, does this mean the future is already fixed? Perhaps so, especially if the block world concept is true. But fixity is not the same as fatalism. The past, present and future can be fixed by many things, including freely willed acts.
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Re: Desert, free will, determinism

Postby TheVat on September 17th, 2018, 12:28 pm 

davidm » September 17th, 2018, 7:08 am wrote:If you're thinking of a conservation problem in terms of the time traveler suddenly appearing in the past, whereas before he was not there, this again would be the second-time-around fallacy. There is only one past, and if the traveler is there, he was always there.


I can see Positor raising the question if we are talking about a technical aspect of physics and COE - but I understand that the chat was focused more on TT as a logic problem. Physically, at the time traveler's destination point, you would have a universe that suddenly has more total matter/energy in it, as if sprung de novo into existence. Personally, I don't see a real problem, as the ledger is eventually balanced again, and we are talking about "laws" like COE as descriptive anyway. The 2nd law of thermodynamics might also be a problem for the TT, but really it's more a suggestion than a law. :-)
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 1:46 pm 

Braininvat » September 17th, 2018, 10:28 am wrote:
davidm » September 17th, 2018, 7:08 am wrote:If you're thinking of a conservation problem in terms of the time traveler suddenly appearing in the past, whereas before he was not there, this again would be the second-time-around fallacy. There is only one past, and if the traveler is there, he was always there.


I can see Positor raising the question if we are talking about a technical aspect of physics and COE - but I understand that the chat was focused more on TT as a logic problem. Physically, at the time traveler's destination point, you would have a universe that suddenly has more total matter/energy in it, as if sprung de novo into existence. Personally, I don't see a real problem, as the ledger is eventually balanced again, and we are talking about "laws" like COE as descriptive anyway. The 2nd law of thermodynamics might also be a problem for the TT, but really it's more a suggestion than a law. :-)


I’ve no objection to discussing the physics of time travel along with the logic, but I’d have to go back and read stuff in that case. :-) It seems to me if you consider an individual as a world tube extended in time as well as space, then I don’t see any conservation problem because the time traveler and his version just being born are just two different temporal parts of the same world tube. I can’t see how this specifically differs from ordinary forward-directed time travel. Consider a closed time-like. But also I think conservation applies locally and not necessarily globally, and energy may not be conserved under GR. Hopefully someone who knows more about physics may come along and sort this.

But also, there is only one past, so you wouldn't actually have a universe with more total matter/energy in it. The time traveler would always have been in the past. So total mass/energy remains the same.
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Re: Desert, free will, determinism

Postby davidm on September 17th, 2018, 2:04 pm 

Physics does meet logic in the Novikov self-consistency principle.

But from the article:

According to the consistency conjecture, any complex interpersonal interactions must work themselves out self-consistently so that there is no paradox. That is the resolution. This means, if taken literally, that if time machines exist, there can be no free will.


This, as I argued above, is wrong, because being able to bring about a logical contradiction is not a valid precondition for any conception of free will.
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Re: Desert, free will, determinism

Postby Positor on September 17th, 2018, 9:01 pm 

davidm » September 16th, 2018, 9:04 pm wrote:A man is born in 2000. In 2018, he steps into a time machine, and travels back to 1999, the year before he was born. In 1999, he steps on a black widow spider.

So let's consider this scenario from his point of view. First he experiences the period 2000-2018, then he experiences going back to 1999, then he experiences the period 1999-2000, then the period 2000-2018 a second time, but as a physically older person with a longer memory. This time he does not experience going back; instead (from his own point of view) he stays on in his second timeframe, and experiences the period 2018-2020 and onwards until his death.
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Re: Desert, free will, determinism

Postby TheVat on September 17th, 2018, 9:47 pm 

So, all through his youth there is this older version of himself who is becoming a very rich man (assuming that he was, at age 18, smart enough to retain some facts about 2000-2018 that would allow the easy accumulation of wealth via shrewd investments, sports betting, etc.). And his older version would also be unable to influence historical events in any way that would alter the first 18 years of his life because he can't change his own past. So, for example, he couldn't invest a fortune in an electric car company that would buy patents, hire engineers, and roll out a Model S before Tesla does. It's possible that causality issues would prevent him from getting rich, or wielding much influence at all? At least until his younger version disappears in 2018.
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Re: Desert, free will, determinism

Postby Positor on September 17th, 2018, 9:55 pm 

davidm » September 17th, 2018, 7:04 pm wrote:being able to bring about a logical contradiction is not a valid precondition for any conception of free will.

Being able to bring about a logical contradiction is not a precondition of free will in normal circumstances. However, it may be an implicit condition of free will in the highly unusual circumstances of time travel. If, on analysis, it turns out that it is an implicit condition in the latter case, then time travel is incompatible with free will.

Compatibilism seems plausible to me where we are dealing with a normal, unknown future set of events, but much harder to accept where a future set of events is completely known. Normally, we cannot form a desire to change the future, because we don't know what is set to happen in the future (assuming it is already fixed). But if we travel back in time, we do know what is set to happen, so we can form such a desire. (Even if we are sure that our desire will be frustrated, we may be interested to find out how it is frustrated.) Having travelled back, we may be free to want to do anything, so in that sense we have free will, but if we know the inevitable outcome it seems an empty kind of free will.
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Re: Desert, free will, determinism

Postby davidm on September 18th, 2018, 10:10 am 

One can concoct an endless number of bizarre time-travel scenarios, as BiV just did, that would take great effort to disentangle. I’ll try to work out BiV’s later. Remember that the thesis on offer is: Time travel is logically possible. Only time travel together with changing the past is ruled out, as a matter of logic. If even one instance can be shown that time travel by itself instantiates a logical contradiction, then TT by itself would be ruled out as a matter of logic.

Of course the time traveler can affect the past. Anything he does in the past, affects past events, including the very act of arriving in the past. But affecting is not changing anything. If he arrived in the past, he had always arrived in the past. If he held up a liquor store in the past, he had always done so in the past. Etc.


Compatibilism seems plausible to me where we are dealing with a normal, unknown future set of events, but much harder to accept where a future set of events is completely known. Normally, we cannot form a desire to change the future, because we don't know what is set to happen in the future (assuming it is already fixed). But if we travel back in time, we do know what is set to happen, so we can form such a desire. (Even if we are sure that our desire will be frustrated, we may be interested to find out how it is frustrated.) Having travelled back, we may be free to want to do anything, so in that sense we have free will, but if we know the inevitable outcome it seems an empty kind of free will.


I disagree.

Let’s break this down. Consider the case of an ordinary time traveler to the future. He considers what he will do tomorrow. He knows in advance that he cannot sprout wings and fly, he cannot bend steel with his bare hands, he cannot will himself to the moon. These things are physically impossible, but do not impeach his free will. It just means free will has physical limits.

But the things listed above are not logically impossible. They will be true at some (logically) possible worlds. There is a world at which you can sprout wings and fly, a world at which you can bend steel with your bare hands, a world at which you can will yourself to the moon. These things are just not true at our world. In modal heuristic they are (logically) possible but (physically) non-actual worlds.

The backward time traveler, knows, similarly, that he can’t do these things in the past. But again, the limit is physical, not logical.

Return to the forward time traveler. He decides that tomorrow, he will kill his grandfather. It is not only physically possible for him to do this, it is logically possible as well.

The backward time traveler decides to travel back in time and kill his grandfather. This is also physically possible, but, unlike for the forward traveler, it is not logically possible. So it seems there is a crucial disanalogy between forward- and backward-directed time travel.

But there isn’t.



Consider more carefully what the backward time traveler actually proposes to do. He wants to go back in time and kill his grandfather, even though it is a fixed fact of the past that his grandfather was not killed. What he is proposing, then, is to both not do and do x at the same time — a logical impossibility.

Look again at the forward traveler. It is logically and physically possible that tomorrow, he should kill gramps. It is also logically and physically possible that he should not kill gramps. What is not logically or physically possible is that he should both kill, and not kill gramps, at the same time.

If the forward traveler wishes to set for himself the exact same task that the backward traveler has set for himself, then he should wish that tomorrow, he will both do and not do x — both kill and not kill gramps, at the same time. This is logically impossible, and thus the undertakings of both the backward and forward time traveler are exactly analogous. Both undertakings are doomed to failure. But again, this fact does not impugn free choice. Indeed, one must choose — kill gramps or not — between two mutually incompatible and jointly exhaustive propositions.
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Re: Desert, free will, determinism

Postby TheVat on September 18th, 2018, 12:57 pm 

Consider more carefully what the backward time traveler actually proposes to do. He wants to go back in time and kill his grandfather, even though it is a fixed fact of the past that his grandfather was not killed. What he is proposing, then, is to both not do and do x at the same time — a logical impossibility.


This gets at what I was saying - a logical universe disallows the contradiction. The mere fact of the traveler's existence means that he had a paternal grandfather who passed specific genes, at a specific moment, to allow conception of his father and then...you get the idea. We don't even need to get into murder here. The backwards time traveler also cannot travel back to around the time right before his father was conceived and bump into his grandfather on the street without subtly altering future events so that the precise moment of insemination is shifted and the specific sperm cell that reaches the grandmother's ovum will be a different one. A consideration of chaos theory would suggest that the small shift in antecedent conditions of a street "jostle" will produce a major set of changes by the time the traveler's own conception is taking place. The time traveler, as we know him, will never have been. He will both exist and not exist.
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Re: Desert, free will, determinism

Postby davidm on September 18th, 2018, 1:24 pm 

Braininvat » September 18th, 2018, 10:57 am wrote: The backwards time traveler also cannot travel back to around the time right before his father was conceived and bump into his grandfather on the street without subtly altering future events so that the precise moment of insemination is shifted and the specific sperm cell that reaches the grandmother's ovum will be a different one. A consideration of chaos theory would suggest that the small shift in antecedent conditions of a street "jostle" will produce a major set of changes by the time the traveler's own conception is taking place. The time traveler, as we know him, will never have been. He will both exist and not exist.


Yes, and it could be that the time traveler travels back in time and jostles grandfather (perhaps inadvertently) in precisely the way needed that makes it come about that the time traveler is born! No jostle, no time traveler. All that matters is the time travel story hold together logically.

There’s a great short story about time travel by Ray Bradbury called “The Sound of Thunder.” It’s a wildly entertaining piece of fiction but totally unsound as philosophy. Same thing with Stephen King’s novel about going back in time and preventing JKF’s assassination. And most other time travel tales as well.

Of course if I go back and time and try to kill grandfather I will fail. But if we are also interested in a logically consistent account, not just of time travel but of free will, we should realize, as explained, that bringing about a logical contradiction is not a pre-requisite of free will. But more, people freely try to do things all the time and fail. Or maybe when the traveler gets to the past with the intention of killing gramps, he won’t even try to kill him and so he won’t fail. He will just change his mind.

As David K. Lewis noted in a classic paper, the possibility of Tim going back in time and killing grandfather is not compossible with a wider set of facts — such as the fact that Tim exists. Why will Tim fail to kill grandfather? “For some mundane reason,” Lewis says, which is also true for our repeated failures to carry out our intentions in ordinary future-directed time travel.
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Re: Desert, free will, determinism

Postby davidm on September 18th, 2018, 1:32 pm 

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Re: Desert, free will, determinism

Postby Positor on September 19th, 2018, 7:35 am 

davidm » September 18th, 2018, 3:10 pm wrote:Consider more carefully what the backward time traveler actually proposes to do. He wants to go back in time and kill his grandfather, even though it is a fixed fact of the past that his grandfather was not killed. What he is proposing, then, is to both not do and do x at the same time — a logical impossibility.

Look again at the forward traveler. It is logically and physically possible that tomorrow, he should kill gramps. It is also logically and physically possible that he should not kill gramps. What is not logically or physically possible is that he should both kill, and not kill gramps, at the same time.

If the forward traveler wishes to set for himself the exact same task that the backward traveler has set for himself, then he should wish that tomorrow, he will both do and not do x — both kill and not kill gramps, at the same time. This is logically impossible, and thus the undertakings of both the backward and forward time traveler are exactly analogous.

I think there is a difference between the two types of logical impossibility. The backward traveller's killing of gramps (call this K1) is logically impossible de re, but the forward traveller's simultaneous killing and not-killing of gramps (call this K2) is logically impossible de dicto.

K2 obviously cannot be a pre-requisite of free will, because K2 is incoherent; it does not denote anything meaningful, so it cannot even be imagined. K1, however, is coherent and imaginable (even though logically impossible in reality). The backward traveller can form a mental picture of himself killing gramps; but he knows he cannot actualize this mental picture, no matter how many times he tries. Can he really be said to have free will if his attempts are 100% doomed to failure?

The backward traveller can, if he likes, ignore the logical impossibility of K1; the thought of K1 is separable from the thought of its logical impossibility. But the forward traveller cannot ignore the logical impossibility of K2; here it is built into the very description.
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