Changing the Past: The Intention Paradox


(a) This is an adaptation of my opening talk for an exhibition by artist Sinead MacDonald this time last year in the Draoicht Art Centre. 

(b) This is about time travel. This is a warning in case you think that is fanciful, and fanciful things don’t interest you.


Many people want to change something in their past. Some of these things – for example, earthquakes – couldn’t be avoided, and so the desire is futile. But others seem tied to our decisions and choices. Let us say you have a time machine. If you could, wouldn’t you want to go back and, at least, try to change some of your decisions and choices — make the other decision, bring about the other outcome? I certainly would; my regrets are not too few to mention.

This seems to be the idea of many people (at least, in the US, and there’s little reason to think it varies in other cultures). Research by the Pew Institute asked the American public what futuristic invention would they most like. 9% of respondents said they would like a time machine. And, according to the show This American Life, most people would like a time machine because they could then go back and change things. For example, a lot of respondents said they would then go back and kill Hitler before he rose to power.

So, it is a common belief that, if you could travel in time, you could also change the past. And this is a trope in lots of time travel stories: Back to the Future, Looper, Primer (although, not Bill & Ted, or Interstellar, or Twelve Monkeys).

However, there is a problem commonly raised about changing the past. At least in some circumstances, you can run into The Grandfather Paradox.

The Grandfather Paradox

The Grandfather Paradox is this (taken from its description by the philosopher David Lewis in the 1970s; the above link also draws from that paper):

1. Tom hates his grandfather. He has a time machine and is a crack shot.

2. Tom travels back in time to before his grandfather meets his grandmother.

3. Tom shoots his grandfather dead.


4. Tom’s grandfather does not meet Tom’s grandmother.

5. Tom is not born.

6. Tom does not get to hate his Grandfather, get a time machine or become a crack shot.

7. Tom doesn’t travel back in time and kill his grandfather.

8. Tom’s grandfather meets his grandmother.


9. Tom is born.

10. Tom gets to hate his grandfather, gets a time machine, becomes a crack shot, goes back in time and kills his grandfather.

11.+ Tom is not born … and so never kills his grandfather…

and so on.

The paradox is not that Tom can kill his grandfather. It is that in doing so he also ensures his grandfather lives (by Tom ceasing to be born, and so not going back). The contradiction is that, in being able to travel back and successfully act on his murderous intentions in the past, Tom both kills and doesn’t kill his grandfather, is born and isn’t born, travels in time and doesn’t travel in time.

The paradox is due to several contradictory statements needing to be true. For example, ‘5’ and ‘9’: Tom is not born and Tom is born. Either ‘5’ or ‘9’ is false or changing the past entails a contradiction.

The Grandfather Paradox has led to some thinkers to conclude time travel is impossible, others that we cannot change the past, others that we can change the past but in doing so we create whole new timelines, etc. etc.

My Opinion

In my opinion, the best outcome is that the past can’t be changed (but time-travel is possible).

I think time-travel is possible, although ludicrously unlikely at our scale. It is a possibility of general relativity but the last account I heard of it (from Kip Thorne) involved either (a) using a planet to tow a black hole around or (b) making a stick-shaped  super-dense star spin several times a second around its own axis. (Good luck with either of these.)

I don’t think appealing to ‘time-lines’, or that kind of approach(alternative universes, etc.), does anything. I don’t think it is really changing the past.

In creating a ‘new timeline’ (in scarequotes because I’m even sceptical that it warrants such a name), this doesn’t prevent the original thing happening. It involves a different situation (via a ‘new timeline’) which resembles a prevention of the original thing. If you can change whatever this situation is, you prevent it in a different timeline. But this isn’t your timeline. If you return to your own timeline, the original situation is still there; if you don’t return to your original timeline, it isn’t — but who cares? You haven’t made the original past change — you’ve just moved to a place which has a different past  (Perhaps this is somewhat equivalent to counterpart theory for timelines.)

Well, you might care if you don’t want to live in a world where it happened. That seems something I imagine many would sympathise with. However, the world where it happened is still there, in the original timeline.   Only if you wipe out the original is it gone; and you don’t do that by creating new timelines. You only do it by altering the original — the very thing the Grandfather Paradox says creates a contradiction.

Still, one might think that the Grandfather Paradox is an extreme situation. Perhaps we can prevent it by only going back to change things that do not entail a contradiction. The kinds of regrets I have are not obviously things which prevent my own existence, for example. So couldn’t I at least go back and change the past in some little way? I’d still survive to the time I travelled back, so I wouldn’t fail to do so, and so fail to make the change.

I don’t think so. And my reasons are intimately related to the intention of trying to change the past.

The Intention Paradox

My thought is this: If you cannot kill your grandfather because of the Grandfather Paradox, then you can’t change anything at all about the past. There is a formally identical paradox which, unlike the odd action of Grandfather murder, is intimately tied to the intention of going back at all.

This is what I call the Intention Paradox of Time Travel:

1. I believe something, call it X, is the case (such as, according to Tom, Tom’s grandfather is a hateful being with a long life).

I intend to change it so that X does not happen (as Tom intends to change it so that his grandfather does not have a long life).

Also, I know how to change things so that X does not happen (as Tom is a crack shot, etc…).

I also have a time machine.

2. I travel back in time to before X happens.

3. I prevent X happening.


4. X does not happen.

5. I do not believe that X happens.

6. I do not intend to change it so that X does not happen.

7. I do not travel back in time and prevent X happening.


8. X happens.

9. I believe that X happens.

10. I intend to change it so that X does not happen, I travel back in time and prevent X happening.

So. X does not happen.

11.+ I do not believe that X happens … and so I do not go back…

and so on.

The parallels with the grandfather paradox should be obvious. The intention to change the past itself prevents its satisfaction. There is no need to have extreme situations for this. It applies to any case where you intend to change the past. So long as your intention is motivated by a belief that something happened in the past, changing that ‘something’ removes the intention. Intending to change that ‘something’ removes itself.

We might insist, of course, that we still exist in the present, unlike in the Grandfather Paradox. However, our existence is not enough to bring about the changed past. We must also intend to go back and change things. Again, it is this intention which is lost in changing the past, just as the agent of such change (Tom) is lost in killing Tom’s Grandfather.

To summarise the point: the Grandfather paradox is a possible paradox generated by being able to change the past. However, if it is a paradox, then there is also an intentional paradox which comes from changing the past. And, unlike the grandfather case, this intention paradox applies to all attempts to change the past (although perhaps not accidental changes, such as Marty’s actions in Back to the Future).

These paradoxes don’t prevent time travel itself. They just prevent changing the past. We might however still go to the past without changing it. So, perhaps, we might just go there to see times which we want to see, and haven’t seen in a while.

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