Tuesday, December 16, 2008

Changing the past

Something I've been wondering about lately -- what modality applies to the impossibility of changing the past? Is changing the past logically impossible, metaphysically impossible, or physically impossible? (If there's some weird physics thing that makes it possible to change the past, please tell me, but I'm not sure how that would work.)

I'm working on a paper that discusses how norms apply to impossible actions, so I'd like to get a better handle on what sort of impossibility would apply to preventing the Civil War or other past-changing actions.


Anonymous said...

The medievals called it "Accidental Necessity," and Alfred Freddoso has a good paper on it:


You may also want to track down Alvin Plantinga's "On Ockham's Way Out," from the journal "Faith and Philosophy."

People in phil religion wonder about it a lot, for reasons having to do with divine foreknowledge and human freedom.

Neil Sinhababu said...

Thanks Tomas! I'll look at that.

Richard Y Chappell said...

I've posted some thoughts here.

Anonymous said...

You should email Peter Vranas at UW-Madison. He just finished a seminar on time travel and has some ideas.

tblogardus said...

Oops, I meant to link you to this Freddoso paper:


The one I linked you to is interesting, but it really just draws out an implication of the characterization of accidental necessity that he gives in this J Phil paper that I just linked you to.

[And just because I'm anal, let me be clear that I didn't actually mean that the medievals called it "accidental necessity." The called it something in Latin that we translate into English as "accidental necessity."]

Anonymous said...

On a slightly less academic note, Neil Stephenson tackles this issue, and also that of changing the future, in Anathem.

If you check out the "Acknowledgments" section of that site, he talks about a number of the people/works he pulled ideas from, most relevantly here in the "Philosophical and Scientific Ideas" subsection, obviously.

I cannot even begin to summarize what it was that Stephenson and his influences were trying to propose, but given that this is not a field that I spend a lot of time thinking about on a regular basis and I still found it fascinating, I have to recommend it.

Neil Sinhababu said...

Whoa, Mark Balaguer is acknowleged on the Anathem site? He just interviewed me last year at the APA!

Anonymous said...

Now you have to read it!

Anonymous said...

I recommend:

Michael Dummett's "Bringing About the Past"

David Lewis's "The Paradoxes of Time Travel"

Anonymous said...

I'm going to say it's a logical necessity. Let P be some proposition about the past. In order to change the past, it would have to be the case at time t1 that P, and at time t2, as the result of my actions, the case that ~P.

This is a contradiction, of course, which is a logical impossibility.

This reasoning applies equally well to changing the future, of course. So we can't change the future either.

This does not imply fatalism, however, which is the thesis that we cannot causally affect the future - which we surely do.

For the possibility of causally affecting the past, see the Lewis and Dummett papers I noted above.