Ferris Jabr, author of a fascinating post on the Scientific American website, observes:
“Why is defining life
so frustratingly difficult? Why have scientists and philosophers failed for
centuries to find a specific physical property or set of properties that
clearly separates the living from the inanimate? Because such a property does
not exist. Life is a concept that we invented. On the most fundamental level,
all matter that exists is an arrangement of atoms and their constituent
particles. These arrangements fall onto an immense spectrum of complexity, from
a single hydrogen atom to something as intricate as a brain. In trying to
define life, we have drawn a line at an arbitrary level of complexity and
declared that everything above that border is alive and everything below it is
not. In truth, this division does not exist outside the mind. There is no threshold
at which a collection of atoms suddenly becomes alive, no categorical
distinction between the living and inanimate, no Frankensteinian spark. We have
failed to define life because there was never anything to define in the first
place.”