Philosophers often talk as if what it takes for a person to persist through time were up to us, as individuals or as a linguistic community, to decide. In most ordinary situations it might be fully determinate whether someone has survived or perished: barring some unforeseen catastrophe, it is clear enough that you will still exist ten minutes from now, for example. But there is no shortage of actual and imaginary situations where it is not so clear whether one survives. (...) Here reasonable people may disagree. There are "fission" cases where each of one's cerebral hemispheres is transplanted into a different head; Star-Trek-style "teletransportation" stories; actual cases of brain damage so severe that one can never again regain consciousness, even though one's circulation, breathing, digestion, and other "animal" functions continue; and stories where one's brain cells are gradually removed and replaced by cells from someone else, to name only a few favorites. (shrink)