In What Time Travelers Cannot Do, Kadri Vihvelin objects to David Lewis's explanation of why a time-traveler cannot kill his own grandpa (From The Paradoxes of Time Travel). To begin, consider the following argument against the possibility of time travel:
The key premise is 2. Suppose Tim entered the time machine on July 4, 2000 and traveled back to July 4, 1940. If, in 1940 Tim kills his grandpa, then his grandpa wouldn't have had a son, and then his son wouldn't have had Tim. So Tim would never have been born. Which means that time wouldn't be alive to enter the time machine on July 4, 2000. We're forced to accept the two following statements: A) Tim entered the time machine on July 4, 2000. B) Tim did not enter the time machine on July 4, 2000. (Because he was never born) A) and B) contradict. This is why some people think that premise 2 in the argument is true. No one can make contradictions true, so no one can kill their own grandpa. Lewis claimed, however, that this argument commits the fallacy of equivocation. This fallacy is committed when an argument depends on the use of a key term and the definition of that key term changes from one premise to the next. Here's a humorous example: 4. Everything that runs has feet. 5. My nose runs. 6. Therefore, my nose has feet. The conclusion is obviously false. The problem with this argument is that the meaning of 'runs' changes from premise 4 to premise 5. This is the fallacy of equivocation. In the case of the time travel argument, Lewis claimed that the term 'can' changes from premise 1 to premise 2. In premise 1, 'can' refers to the ordinary sense of 'can' in which someone has what it takes to kill their grandpa: they have the training, the rifle, the ideal settings, clear vision, the ability to pull a trigger, etc. But in premise 2, the sense of 'can' changes to something else. We can see this by imagining Time-Traveling Tim the moment before he attempts to kill his grandfather. He has the training, the rifle, clear vision, etc. So he obviously 'can' kill his grandpa in the sense from premise 1. But if we interpret premise 2 like this, then it is false. So in order for the argument to work, it has to slide into another sense of 'can'. But when we do this the argument commits the fallacy of equivocation. So either way, the argument fails. Lewis thinks that premise 2 is indeed false. He thinks that time travel is possible. But he also thinks that Tim doesn't actually kill his grandpa in any time travel scenario. This is how he avoids the problem of the contradiction between A) and B). He says that the most likely reason is that every time Tim tries to kill gramps, something goes wrong. Either the gun jams, or he misses, or the cops stop him, or his grandfather falls right as the shot is fired, etc. So Tim 'can' kill his grandfather in the ordinary sense, but he won't because he's still alive in 2000. Now back to Vihvelin. Vihvelin thinks that Lewis is wrong. She thinks that premise 2 actually uses the same ordinary sense of 'can' that premise 1 uses. But she thinks that premise 1 is false. Her explanation for this is that we should define what we can and cannot do on what we could possibly do, keeping everything about us as similar as we can. She explains this by the use of a special kind of conditional called a 'counterfactual'. A counterfactual is an 'if, then' statement in which the part after the 'if' is false. Here are some examples: C) If I had bones made of adamantium, then I would be heavier than I am. D) If I was from the planet Krypton, then I would be just like Superman. E) If I was 7 feet tall, then I would have played college basketball. All of these have false antecedents (the part after the 'if'). They are "counter to fact". That's why they are called counterfactuals. They are also all true. Adamantium is a very strong metal that is supposedly heavier than human bone. All aliens from the planet Krypton are supposed to be more powerful than Earthlings. And I was a pretty good basketball player, so if I was that tall, then I definitely would have played in college. We can define what we can and cannot do by using counterfactuals like these. Consider these two: F) If I try to sink a 3-pointer, I would or at least might succeed. G) If I try to slam dunk on a regulation hoop, I would or at least might succeed. F) is true (I'm still a pretty good shot after all these years!). But G) is false. Even in my prime, I was never able to dunk (I'm too much of a white boy!). So we can say that I 'can' sink a 3, but I 'can't' dunk. Alright, back to Vihvelin. She thinks that we should define 'can' based on what can possibly happen and we know what can possibly happen based on what would or might happen. Consider this counterfactual: H) If Tim had tried to kill his grandpa, then he would or might have succeeded. Is H) true? Vihvelin reminds us of what Lewis said: in order to avoid a contradiction, we have to say that Time-Traveling Tim always fails whenever he tries to kill gramps (either the gun jams, or he misses, or he chickens out, etc.). But if he always fails, then there is no possible way that H) is true unless there's some kind of miracle. But this is exactly like F)! I cannot dunk. Obviously. Would you believe me if I said, "I can dunk, it will just take a miracle!" No. You'll rightly call me out as a liar. I can't dunk. Therefore, Vihvelin also says that we should say that Tim can't kill his grandpa. |
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