Longevity is perhaps the best single measure of the physical quality of life. (If you're dead, there's little you can do to be happy.) This is a precious offering from science to humanity -nothing less than the gift of life.
Sagan, C. and Druyan, A. (1997) The Demon-Haunted World: Science as a Candle in the Dark. Reprint edition. Westminster: Ballantine Books.
