In its broadest sense, the Theory of Evolution is a very powerful explanatory tool. Many people admire its explanatory power, especially those with a scientific or technological bent. The most recent example of this admiration I've encountered is an article by Brett Glass, which he opens with a casual remark about the human immune system, describing it as "honed by millions of years of evolution". Next time you see a throwaway remark of this kind, I'd like you to think about the following related observations.

The broad and nonspecific way in which the Theory of Evolution is invoked here suggests a process of stepwise refinement, driven entirely by natural forces, progressing in a chaotic (but ultimately "upward") manner over vast periods of time. In this instance, the vast periods of time were mentioned specifically, emphasising the amount of evolution that has taken place. The actual mechanism of evolution is handwaved in this broad description: the only hard constraint is that the process be natural, as opposed to supernatural or miraculous — the primary point being one of differentiation with theories involving an intelligent creator.

The explanatory power of this broad and nonspecific Theory of Evolution is immense. Any clever mechanism can be explained by appeal to sufficient quantities of the process. Brett Glass sees a "wondrous security system" in the human immune system, and attributes its wondrous properties to "millions of years of evolution". Although he doesn't do so in this particular essay, he could also explain the imperfection of the human immune system by exactly the same process. After all, the process itself only ever produces something which is "good enough", rather than perfect. So no matter what you see in nature, be it cunning implementation or dismal waste, evolution can explain it.

And that's the problem.

If a theory can explain anything, then it's not evidence for anything. Or more precisely, if you make an observation about the real world and declare it "compatible with evolution", then this is only evidence for evolution to the extent that we could have observed something else that was evidence against it. If a theory is compatible with any eventuality, then it's not a theory in a scientific sense, but rather what I would call a Weltanschauung — a particular way of viewing the world. And, in this sense, Brett Glass is making appropriate use of the all-explaining Theory of Evolution: rather than citing the immune system as evidence for the theory, he's simply saying that, in his view of the world, the immune system is (like everything else) the product of this natural process we call evolution.

So don't mistake a reference to evolution of this sort as any kind of evidence for the theory itself. It is, rather, strong evidence for the proposition that the person in question holds a certain metaphysical belief about the nature of the universe. To put it another way, the fact that Brett Glass links the immune system with evolution tells us more about the metaphysical framework employed by Brett Glass than it does about the actual connectedness of evolution and the immune system.

In all fairness, I should point out that broad Theories of Creation are just as all-explaining as the Theory of Evolution. Where we see cunning implementation, a creationist will probably cite "intelligent design"; where we see dismal waste, a Christian creationist is likely to cite the fall from perfection in Eden. Once again, it's unlikely that you could come up with any scenario which is truly incompatible with the theory (or Weltanschauung, as I would have it), to the extent that it made the theory obviously unreasonable.

Although I'm a Christian and a creationist, I'm not inclined to open an essay in the way that Brett Glass did, citing an aspect of his Weltanschauung. In the first instance, I like to treat my Weltanschauung as arbitrary rather than granted truth — just because I believe it doesn't mean anything special in the overall scheme of things. In the second instance, most people don't subscribe to the creationist view, and I'm well aware that starting an essay with a reference to creation is likely to cause the evolutionists in the audience to switch off.

I hope you'll be reminded of these ideas the next time someone states their Weltanschauung in the way that Brett Glass did. As an aside, the article in question actually draws analogies between the immune system and the way in which society responds to things like the September 11 atrocities, and it makes some good points. In general, I'm predisposed to think that persons with a given name of "Brett" must have something going for them, but this is just an element of my Weltanschauung, so don't give it too much credence.