“For a merely conscious being, death is the cessation of experiences, in much the same way that birth is the beginning of experiences. Death cannot be contrary to an interest in continued life any more than birth could be in accordance with an interest in commencing life. To this extent, with merely conscious beings, birth and death cancel each other out; whereas with self-aware beings, the fact that one may desire to continue living means that death inflicts a loss for which the birth of another is insufficient compensation.”