Friday, January 13, 2012

Life: You Know It When You See It

Carl Zimmer, who blogs at The Loom, is interested in definitions of life. His latest essay highlights a definition proposed by Edward Trifonov who says that life is: "self-reproduction with variations" [Can A Scientist Define “Life”?].

It didn't take Sean Carroll (the physicist) very long to see one of the main problems with this definition; namely that Sean isn't alive! [Do I Not Live!].

It's true that we should not restrict our definition of life to things that can self-reproduce. It's also true that we should not restrict our definition to things that reproduce badly (e.g. mutation/variation). I can easily imagine living things that could reproduce perfectly—they just wouldn't evolve by any mechanism we currently recognize.

Life is like pornography [I know it when I see it]. In the words of US Supreme Court Justice Potter Stewart,
I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"]; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.


  1. Very few things are more pointless than arguing about precise definitions of abstract entities.

  2. Part of the definition problem might be that 'life' is both a property of individual entities (they are 'alive') and a phenomenon (the aliveness of many kinds of entities). More generally, I agree with DK. Like many other terms humans have created, the term 'life' is least useful when it needs to be very precise. In such instances it's better to dispense with 'life' and use terms that better capture whatever it is we want to talk about (metabolism, evolution, ...)

  3. I think a definition of life that's basically "I know it when I see it" is going to fail miserably when applied to artificial intelligence. (In fact, that was sort of the point of Alan Turing's famous paper introducing the Turing Test.)