J: How do you define life?
A: Life is the opposite of death.
J: So death is the absence of life?
A: Take it easy. I try not to think about such things.
J: Why not?
This problem of defining “life” and “death” in a circular fashion prompted me to think about how I would define them, and although I liked the “death = absence of life” definition, I realized that I didn’t readily have a good definition of “life” handy. So I decided to look up some definitions written by others. There seem to be at least three generally-agreed upon characteristics that an entity must exhibit in order to be considered “life:”
- Metabolism (the production and consumption of energy)
- Adaptation to environment
Some definitions also mention “growth,” although it doesn’t seem reasonable to deem something “dead” once it has stopped growing. It also seems a bit troublesome to include “reproduction” in the definition, since this seems to prohibit us from including children. You could say that “reproduction” refers to the possibility of reproducing at some time in the future, but the existence of things like worker ants cause a problem for that definition as well. The solution here is to say that life is inherited, and thus these non-reproducers are still lifeforms since they are a result of the reproduction processes of other lifeforms.
It seems to me that this definition problem causes trouble for both evolutionists and creationists. At what point during evolution did “life” begin? What sort of chemical process could be responsible? How exactly did God breathe “life” into Adam? How could that possibly work physically, chemically, or biologically? All the expert physicists, chemists, and biologists who read my blog are welcome to chime in at this point. 😛
Apparently this question of defining “life” is still somewhat of an open problem for biologists. At this point, it seems to be one of those “I can’t define it, but I know it when I see it” sort of things. If you’re interested in reading more about this, the Wikipedia article is a good place to start (check out the links at the end of the article).