By Josh Gabbatiss
Most of us probably do not need to think too hard to distinguish living things from the “non-living”. A human is alive; a rock is not. Easy!
Scientists and philosophers do not see things quite this clearly. They have spent millennia pondering what it is that makes something alive. Great minds from Aristotle to Carl Sagan have given it some thought – and they still have not come up with a definition that pleases everyone. In a very literal sense, we do not yet have a “meaning” for life.
If anything, the problem of defining life has become even more difficult over the last 100 years or so. Until the 19th Century one prevalent idea was that life is special thanks to the presence of an intangible soul or “vital spark”. This idea has now fallen out of favour in scientific circles. It has since been superseded by more scientific approaches. Nasa, for instance, has described life as “a self-sustaining chemical system capable of Darwinian evolution”.
But Nasa’s is just one of many attempts to pin down all life with a simple description. In fact, over 100 definitions of life have been proposed, with most focusing on a handful of key attributes such as replication and metabolism.