Introduction
The first question in the philosophy of time travel is whether travel to the past is possible or whether instead it conceals some hidden paradox. The pre-eminent contribution to the philosophy of time travel is David Lewis’s engaging discussion ‘The Paradoxes of Time Travel’ (Lewis Reference Lewis1976). Lewis’s overall conclusion is that travel to the past is possible and that all apparent paradoxes can be resolved satisfactorily. The most famous such paradox has come to be known as the Grandfather Paradox. Thus: can a man travel to the past and kill his own grandfather? In Lewis’s thought-experiment we are told that Tim’s grandfather died in 1957. Tim’s father was born in 1922. Tim built a time machine and travelled back to 1921 with the intention of killing Grandfather in that year.
Having arrived in 1921, Tim buys a rifle and tracks the route of Grandfather’s daily walk. On one quiet, sunny day, with Grandfather firmly in his sights, Tim takes aim, pulls the trigger, and … ? Since Grandfather lived until 1957, Tim’s attempt fails. Why does it fail? According to Lewis: ‘For some commonplace reason. Perhaps some noise distracts him at the last moment, perhaps he misses despite all his target practice, perhaps his nerve fails, perhaps he even feels a pang of unaccustomed mercy’ (Lewis Reference Lewis1976: 150). Any attempt by Tim to kill Grandfather will be prevented in one way or another. These preventers are quite ordinary and contingent in nature; no Hand of Fate or Force of Logic is required. This is one plank of the standard resolution of the Grandfather Paradox.
But there’s another plank. At a more abstract level, it seems that Tim not only does fail but must fail. Tim isn’t attempting to kill just anyone. He is attempting to kill Grandfather prior to Father’s conception. That is, Tim is attempting to undermine a condition for his own existence. As Lewis notes: ‘No Grandfather, no Father; no Father, no Tim; no Tim, no killing’ (Lewis Reference Lewis1976: 152). Since no one can make it the case that they never existed, Tim must fail in his grampicidal endeavour.
A Puzzle?
The standard resolution of the Grandfather Paradox thus has two components: a contingent component (Tim’s grampicidal attempts are subject to preventers of an ordinary kind) and a necessary component (Tim’s attempts must fail since he can’t undermine a condition for his own existence). Yael Loewenstein recently pointed out a seeming oddity in the standard account (Loewenstein Reference Loewenstein2022). Why the need, she asked, for the contingent plank if we have the necessary one? Consider a different example. It’s impossible to paint a wall red and green all over at the same time. Pointing this out suffices to explain why any attempt so to paint will fail. No other explanation is needed or even possible. There’s no space for a preventer to explain why the attempt of some deluded artist fails. Nothing prevents him, but only because the notion of prevention gets no traction here. Nothing he daubs on a wall could possibly count as a case of painting it red and green all over. And that’s all that can or need be said.
Why then isn’t the same true in the Grandfather case? If Tim’s attempts to kill Grandfather must fail, pointing that out should be explanation enough. Why the need to talk about near misses, failures of nerve, pangs of mercy, and so on? There ought to be neither need nor room for such preventers. And yet it seems that there is.
‘Having arrived in 1921, Tim buys a rifle and tracks the route of Grandfather’s daily walk. On one quiet, sunny day, with Grandfather firmly in his sights, Tim takes aim, pulls the trigger, and … ?’
The Puzzle Resolved
But is there really a puzzle here? Tim’s attempts at grampicide are guaranteed to fail, yet each failure has an ordinary, commonplace explanation. Is this mixture of necessity and contingency paradoxical, as Loewenstein thinks? Arguably not. Baron and Colyvan cite the example of the Konigsberg Bridge puzzle (Baron and Colyvan Reference Baron and Colyvan2019: 249). Euler famously proved that it’s impossible to cross all seven bridges in Konigsberg without crossing one bridge twice. Suppose that the somewhat dim Tim, prior to embarking on his time travel adventure, set himself the ‘Konigsberg Challenge’: to cross all seven bridges without crossing one twice.
‘Tim is attempting to undermine a condition for his own existence. As Lewis notes: “No Grandfather, no Father; no Father, no Tim; no Tim, no killing”…’
Tim’s failure to complete this task is guaranteed: he must fail. But there will be a commonplace explanation for each failed attempt (he slipped on a banana skin, he gave up and went to the pub, he inadvertently crossed the same bridge twice, etc.). Tim must fail the Konigsberg Challenge, yet each failure has an ordinary explanation. There is no mystery to this conjunction. Similarly, it might be said, there is no mystery in the case of Tim, the ever-thwarted would-be Grandfather assassin.
Conclusion
Lewis never told us the amusing end to Tim’s story, so let me do so here. After a series of comical attempts at assassination, Tim and Grandfather meet up. Over a few beers, Tim tells Gramps his whole sorry tale. The dialogue between the smart Grandfather and the not so bright Tim then goes as follows:
Grandfather: Tim, I was just wondering, do I ever get a criminal record?
Tim: Gosh no. We had a joke in our house that old Gramps never got so much as a speeding ticket!
Grandfather: Oh, that’s good to know.
Grandfather then shoots Tim dead.
Our title is therefore apt. Consider the delightful ambiguity in phrases such as ‘Grandfather’s murder’ and ‘the murder of Grandfather’. Richard Hull’s enjoyable novel Murder of My Aunt (Reference Hull1934) plays on this ambiguity.
Thanks to Alan Hajek, Daniel Stoljar, Yael Loewenstein, and Stephen Law, for useful feedback.
