Another metaphysics post, this time getting away from Kripke and language:
One example introduced on the first day of the metaphysics class I'm taking was of a 17 year old slacker who goes off, spends three years in the military, and as a result changes significantly in character, is more disciplined, etc. When he gets back, he reports, "I'm a different person now." Philosophers, being philosophers, may be inclined to ask in this situation whether this might literally be true.
It sounds silly, but I've thought of another thought experiment that suggests something significant may be going on here. Imagine a person is kidnapped by time travelers, whether from another planet or a future Earth it matters not. In addition to time travel, their technology includes great medical and genetic technologies, which allow the subject to be kept alive indefinitely. Over a course of ten thousand years, he goes on many adventures, entirely forgetting his original life for all practical purpose, and his genetic code is slowly altered, one tiny insignificant bit at a time, until it is unrecognizable. Maybe he has some vague ideas about what it was like to live in his home milieu, and maybe his genetic code could be recognized as originally human, but both memories and genetics have been altered (slowly, over the course of ten thousand years, remember!) to the point where he could never be identified with a single individual.
To complete the thought experiment, imagine that somehow the time traveling technology finally places him back in his home setting, an hour or two after the abduction. Would it make any sense whatever for the subject's previous friends and family to treat him as in any way the same person? I think not. I dare say he would not be the same person. This is in spite of the fact that the example is different only in extent from the military case, and I have stipulated that the change is completely gradual. It is interesting to think that the ex-slacker will likely not interact with many of his old friends in quite the same way, especially if they themselves have remained slackers. The time traveler is only a more extreme form of that.