Our question is not about what the world would be like if that adult-stage tried and succeeded in killing this baby stage. Our ques- tion is about what that adult stage can do, and that is a question about what whether that adult stage would have succeeded if it had tried to kill this baby stage. And I think the answer isclearly "No". It is false that if that adult stage had pulled the trigger, then the adult stage and the baby stage would have been stages of two different persons. Any world like this is far less like ours than a world where that adult stage tries, but fails to kill this baby stage. To see this, consider what the world would have to be like for it to be true that if that adult stage had pulled the trigger, then that adult stage and this baby stage would have been stages of two different persons. Let's pick a specific occasion- say Sept. 3, 1960, when Baby Suzy is one year old and adult Suzy is thirty-six and is standing a few feet from Baby Suzy, gun in hand and murder on her mind. As a matter of fact, that adult stage and this baby stage have matching DNA and are linked by chains of causal dependencies stretching from 1960 to 1995 and then back again to 1960. We are being asked to suppose that if that adult stage had pulled the trigger, then this baby stage would have died and these causal chains would not have obtained. But then how would that adult stage have come to be there in 1960, gun pointed at this baby-stage with matching DNA? There seem to be just three possibilities: Either that adult stage would have miraculously come into existence out of thin air, or the DNA of some actually existing person would miraculously have changed so that it matches the DNA of that adult stage, or the world would have included another baby stage with DNA that matches the DNA of that adult stage and thirty-six years of the appropriate causal connections would have linked this extra baby stage to that adult stage. But worlds like these are far less like ours than worlds where the gun jams, or the bullet misses, or... and that adult stage fails to kill this baby stage. So if that adult stage had tried to kill this baby stage, the stages would still have been stages of the same person, and the attempt would have failed.
WHAT TIME TRAVELERS CANNOT DO 327 Objection: Look, suppose that over on Putnam's Twin-Earth, Suzy's doppelganger gets into a time machine to kill her baby self, but wires get crossed and she materializes right beside our Baby Suzy. You agree that Twin Suzy can kill Baby Suzy. Yet you are committed to saying that Suzy cannot do what her atom-for-atom duplicate can do. Reply: I am committed to that, and I agree it seems odd. 4 It seems odd because, ordinarily, when we are considering what someone is able to do right now, the past is irrelevant. Or rather, the past is irrele- vant except insofar as it affects current present-tensed properties. Of course, Suzy can't swim if she hasn't taken lessons, but the lessons enable her to swim only because of the difference to the way her brain is set up now. It's her current brain set-up that gives her that
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- Spring '11