“What happens when an animal or person dies? Something seems to have departed--something like a vital spark that makes the difference between life and death. In the nineteenth century, philosophers believed that there really was such a thing and called it the élan vital, or vital spirit. But when twentieth century science began to unravel the mysteries of how living things work and reproduce, the idea was abandoned and people now accept that there is nothing more to being alive than complex, interrelated, biological functions.”