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.
