No question in science is more fundamental. No question is more difficult to answer. It is easy to recognize life when we see it, but devilishly hard to say what it is.
The biologist Lynn Margulis and co-writer Dorion Sagan tackled the question in a book called "What Is Life?" They proffered several definitions:
"A material process, sifting and surfing over matter like a strange, slow wave."
"The watery, membrane-bound encapsulation of spacetime."
"A planetary exuberance."
All of which are lovely metaphors, but none of which get us any closer to the ineluctable heart of the mystery.
What is life? How does one define a flight of wild geese, a Mozart symphony, love?