Why don't we consider fire alive? What is life?
OK, I know the answer to this, I just want to hear some reasons. 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?
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 they know it when they see it, there's 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:
It's the name of a magazine. I'm allowed one inane joke per column. This is it.
Living things contain reproducible hereditary information. This is the genetic definition. I avoided mentioning DNA, nucleic acids, chromosomes, and such so as not to limit this 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 biologists don't buy that. 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 the environment, etc.
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.
Life reverses local entropy. Popularized by Isaac Asimov. In lay terms, life reverses the default trend toward ever-greater disorganization. Yeah, I know: Asimov must never have 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.
Admittedly that idea isn't much help in deciding what's alive and what ain't. The chemical reactions occurring for the universe's first few billion years led inexorably to our teeming world, yet no one would describe them as life. But so what? The claim is that there is a powerful antientropic force in the universe (at a certain level of organization, it's called natural selection), of which we are the latest and coolest manifestation. I'm not saying it's as cuddly a thought as a benevolent Creator. But it beats waiting for things to just sputter out.
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."