The best metaphysical definition of a living thing is a freely acting thing. Any thing that seems to exhibit free will seems to be alive. It’s that simple. Hence, my fingernails are not alive. For their activity is purely mechanistic. But my fingers are alive, for their activity is unpredictable. They might continue to resign themselves to typing this message, or give up and go play the piano, or reach out and slap you. One can never tell, not even I, what my fingers will do next.