Suppose every night while you slept someone came into your room and replaced your body parts, one cell at a time. At the end of ten years, she has replaced every cell in your body with an exact duplicate (so you’d have no loss of memory, strength, etc. as a direct result of her work) and destroyed the cells she’d removed.
Luckily, you have a security camera in your bedroom, and at the end of ten years you review the tapes and see that you have been completely replaced, or you are a complete replacement (the wording is inessential here, except perhaps to how you feel about yourself.) You have the culprit arrested.
She is charged with breaking and entering and assault, and probably some other crimes. But can this person be charged with murder? After all, she did completely destroy your body.
Could we charge her with murder if it took her 40 years to replace your cells? One year? One evening? One hour? If there is a cutoff point for when this is murder, how is it arrived at? If it’s murder if done in an hour, but not if done over 40 years, how do we make the distinction?
Further, suppose that before the replacements began, you had committed a murder. Are you, or whatever entity that exists at the end of the replacement procedure, guilty of that murder? Again, does it matter if the procedure took 40 years or one night?
Popular Metaphysics 