This weekend the New York Times gave
Philip K Dick the writeup he deserves, at long last. While I could ramble about Dick, as I love good sci fi and he is one of the pulp kings of sci fi, I would rather introduce a little disturbing mindf**k that comes about when one marries two of my interests, analytic philosophy & sci fi. The other day there was a post about
killing evil figures of yesteryear that got me thinking about time travel and logical paradoxes. Here’s what I came up with:
I have been thinking about the grandfather paradox, a logical paradox about time travel, famously explored in
David Lewis‘s paper,
The Paradoxes of Time Travel.

The paradox employs a basic modus tollens form that allows the negation of the statement, “time travel is possible”. It is based around the simple principle that it can never be the case that grandfather both lives until a certain date, say 1957, and does not live until 1957. The argument that backs up the idea that no time traveler can kill his grandfather is an argument based around counterfactuals.
(click on either of the arguments for the pdf they are taken from) 
The important thing to note at this point is that, no matter how one feels with respect to whether or not they can kill grandfather, “In no sense of ‘can’ is there something [one] can do, such that if [one] had done it, contradictions would have been true.”
Naturally this strikes one as bizarre. It clearly seems that, upon arriving in the past and preparing appropriately for the murder, one could succeed in pulling it off. While there are certain probabilities of failure with respect to local conditions (for example a jamming of the rifle, a missed shot, a strong breeze at the last minute) governing the success of the murder attempt, there also seems to be what another philosopher, Lockwood, describes, in a term borrowed from physics, as a ‘boundary condition’. A boudary condition would be some action limiting condition that prevents Tim from persuing certain lines of action. This boundary condition involves taking different facts into account, namely facts about how long grandpa will live and facts about the impossibility of contradictions, which limit Tim’s actions by way of certain ‘global constraints’ on his actions which perhaps manifest themselves as ‘local constraints’ in the way of a gun jam et al. In fact, it seems that, no matter how many times Tim tries to kill his grandfather in 1921, he will fail at every attempt, given the fact that grandpa lives until 1957.
Truly it is bizarre to think that there may be certain logical constraints on our acts. It is difficult to admit that our actions our limited by the fact that they must be consistent with certain facts about the future, whether or not we have epistemic access, or the ability to know about, these facts. On the other hand, time travel necessarily employs a certain
‘tenseless’ concept of time, as many philosophers who have thought about it have concluded. These ideas certainly are a bull in the chinashop that is the leyman’s concept of free will, but that is another discussion. If we concern ourselves with recent advancements in physics,
they tell us the following:
Physics… does have one lesson to impart to the free will debate; a lesson about the relationship between time and determinism. Recall that we noticed that the fundamental theories we are familiar with, if they are deterministic at all, are time-symmetrically deterministic. That is, earlier states of the world can be seen as fixing all later states; but equally, later states can be seen as fixing all earlier states. We tend to focus only on the former relationship, but we are not led to do so by the theories themselves
Now this is getting bizarre. It seems, on considering the forwards and backwards determined nature of particle movement, acts, and other physical events, with respect to time, that they are somehow constrained in more than just a logical sense. Tim the time traveler is not alone, you are unable to successfully perform certain acts, intuitively, due to the fact that there are certain constraints on what one can and can’t do with respect to certain facts about the future, whether or not we have any form of access to those facts. In short, its silly to say that you ‘can’ do something if it is something other than what you do, in fact, end up doing. It seems that physical ability, the laws of nature, and various other local considerations about whether or not one can treat the ability to do something as a ‘live’ or resonable option do not actually bear on whether or not that action is something that one actually ‘can’ do. It seems local constraints may only be such insofar as they must jive with ‘global constraint’ of the system one functions within.
What do you think?