““Life is easy to identify but remarkably difficult to define. Too tight a definition excludes what looks like life and too loose captures too much. The capacity to self-replicate is a component of the definition, but not without its problems, as a mule is alive but sterile, and computer software can replicate itself, but we do not, in all honesty, think of it as being alive. It might be tempting to ascribe livingness to an entity that has emerged by evolution, but that would exclude the first living entity and any that we might synthesize from scratch in future. Organisms are organized structures; but so is an integrated circuit. Organisms are organized structures built and sustained by the flux of energy through their interiors and its dissipation into the surroundings; but so are the patterns of convection that can arise in heated liquids and, indeed, the atmosphere, to give rise to the weather: think tornado and hurricane. All known organisms are built from compounds of carbon; but if we succeeded in building a replicating, conscious, self-sustaining, energy-dissipating entity from silicon, would we deny that it was alive? Is a virus alive?””
