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Why isn't fire alive? It breathes oxygen. It eats wood. It reproduces (sort of). What defines life? How do we know it's not an advanced non-carbon-based life form? On that same note, why isn't a rock alive? If a rock grew at a nanometer every million years, how could we possibly study something like that? -- Gwidion 15, via AOL
You know, Gwidion, a lot of people reading this are thinking, "That's what you get for riding a motorcycle without a helmet." Not me. Truth be told, though everybody thinks he know it when he sees it, there is no widely agreed upon definition of life. In fact, fire is sometimes used as an example of something that obviously isn't alive but nonetheless exhibits many functional characteristics of living things, e.g., metabolism, growth, reproduction, and so on. But if the functional definition (it's alive if it acts like it's alive) won't cut it, what will? Here are a few other definitions of life:
(1) It's the name of a magazine. (I'm allowed one inane joke per column, and this is it.)
(2) Living things contain reproducible hereditary information. This is the genetic definition. You'll notice I avoided mentioning DNA, nucleic acids, chromosomes, and such, so as not to limit this definition to life as we know it on earth. Yet this definition is still open to criticism. Some people argue that a machine could contain reproducible hereditary information, but we wouldn't consider it alive. Most scientists would counter, "Why not? If we accept the possibility of artificial intelligence, why not artificial life?"
A more serious objection is that by this definition, a virus is alive. A lot of biologist types don't buy this. A virus is basically a chunk of DNA or RNA (or computer code, for that matter) that succeeds in reproducing itself. But it's not a cell, which many consider the fundamental unit of life, and it doesn't do the things cells do, such as metabolize, react to their environment, et cetera.
(3) Life is an illusion. Now I'm really starting to feel that six-pack. Let's set aside the question of sentient life to avoid arguments about the soul. It seems obvious that at some level all we see about us, living or otherwise, is merely a manifestation of chemical reactions and the laws of physics. Chemists implicitly accept this mechanistic idea, defining "organic chemistry" (whose nominal subject is the chemical reactions underlying life) as anything having to do with carbon. In short, life is an arbitrary distinction.
(4) Life reverses local entropy. Popularized by Isaac Asimov. In lay terms life reverses the default trend toward ever-greater disorganization. Your initial reaction may be: Asimov must have never had children. Still, this one's got a certain appeal. In contrast to, say, fire, which in its uncontrolled form is one of your more basic entropic phenomena, life is a creative force. On the other hand... well, think about it. At the low end of the entropic (i.e., organizational) scale, we have the primordial hydrogen soup whence arose the universe we know; at the other end we, have the Microsoft Corporation. No one would describe the chemical reactions occurring for the universe's first few billion years as life, yet somehow we wound up with Bill Gates. This suggests two things. First, it may be hopeless to define life in a nontrivial way. Second, there seems to be a powerful antientropic force in the universe (at a certain level of organization, some call it natural selection), of which we are merely the latest -- and so far the coolest -- manifestation. (Actually, I am the coolest manifestation. I just mentioned Bill Gates to be polite.)
One more thing. When this question came up on The Straight Dope message board the other day, one confused philosopher attempted to settle the issue by saying, "Cogito ergo sum [I think, therefore I am]; fire no cogito." Another wit replied, "If [cogito]'s the litmus test, we're going to lose most of California."
Is there something you need to get straight? Cecil Adams can deliver the Straight Dope on any topic. Write Cecil Adams at the Chicago Reader, 11 E. Illinois, Chicago, IL 60611; e-mail him at firstname.lastname@example.org; or visit the Straight Dope area at America Online, keyword: Straight Dope.