Carl Sagan (1934-1996) offered several definitions of
life:
Physiological --- Idea: Living organisms
metabolize, move, eat, etc. Problem: Cars.
Metabolic --- Idea: Living organisms produce
energy from their environments. Problem:
seeds and spores don't metabolize but
are living things (well, sort of);
and fire obeys that definition, but is
not living.
Biochemical --- Idea: Living organisms
have a certain, specific, unique
chemical make-up. Problem: Do prions
count? How about man-made substances?
Genetic --- Idea: Living organisms
reproduce and pass on hereditary information
to offspring. Problem: Mules.
Thermodynamic --- Idea: Living
organisms increase order in their
environments. Problem: no problems ...?