 What matters in identity? What is the practical importance of facts about our persistence? Why does it matter? What reason have you to care whether you yourself continue to exist, rather than someone else just like you existing in your place? Imagine that surgeons are going to put your brain into my head and that neither of us has any choice about this. Suppose the resulting person will be in terrible pain after the operation unless one of us pays a large sum in advance. If we were both entirely selfish, which of us would have a reason to pay? Will the resulting person—who will presumably think he is you—be responsible for your actions or for mine? (Or both, or neither?)