“Our actual lives, including our values, our social relations, our self-conceptions, and many of our concepts, are pervasively shaped both by the knowledge and by the fact that we will someday die - that we are subject to extreme temporal scarcity. There is no reason to think that, if we were immortal, the same things would continue to matter to us. We have little or no idea what, if anything, would matter to immortal beings, or even how such beings would think of themselves.”