Biologists with a predilection for theory have tried-and largely failed-to define what it is that makes something a living thing  Organisms take in energy- providing materials and excrete waste products, but so do automobiles  Living things replicate and take part in evolution, but so do some computer programs  We must be open to the possibility that there are living things on other planets  Ttherefore, we will not be successful in defining what it is that makes something a living thing merely by examining living things on Earth--the only ones we know
Trying to do so is analogous to trying to specify
the laws of physics by using pure mathematics