Life is a diagnosis, not a definition

Over on Carl Zimmer’s blog, evolutionary biologist David Hillis gives the most satisfying answer I’ve heard for why we don’t have a good definition for life. I have trimmed and revised it below.

It is not sensible to define life using a list of characteristics. It is only sensible to “define” Life (capitalized because it is a formal taxon) by pointing to it, noting when and where it began, and following its lineages from there.

You could define a class concept called life (lowercase because not a formal taxon), but then that concept would clearly differ from person to person. So, I’d say that I can point to and circumscribe Life, and that that is the appropriate way to “define” any biological taxon. A list of its unique characteristics is then a diagnosis, rather than a definition.

Defining Life is like defining other particular historical entities. We don’t “define” LeBron James or Detroit by listing out their attributes. Instead, we point to their origin and history. The same should be true for Life. If we ever discover Life on another planet, we’ll have a new origin and history to point to.