ID is what is not testable, because no mechanism or characteristic outcome is proposed (aside from "it looks complex sometimes, other times it's simple, whatever the designer decides").Interestingly, Behe goes into more detail in his book on this issue. As I've said already, Behe accepts both common descent and natural selection. What he takes issue with is whether random mutation, as a key part of darwinism, is up to the job of explaining the complexity of life.
What's the alternative? He points out that if there is a "designer" who is able to fine tune the cosmos in such a way as to make it fit for habitation, there's no inherent reason that it shouldn't be possible for this designer also to arrange events - either at the time the universe appears or at a later stage - to bring about other goals that he wishes.
This line of argument reminded me of Schaeffer's argument that propositional revelation is not nonsense. I'm not sure it actually leads any further than deism - but then, I would suggest that Romans 1 argues that this is pretty much as far as general revelation would take you anyway, which is fair enough. Behe argues that further discussion about the nature of the designer is a matter for philosophy and theology - again, I'm pretty happy with that.
"So," the opponent of ID then answers, "ID is unfalsifiable. Behe is saying that the events that make up the evolutionary history of life could be interpreted as 'natural' - the inevitable outcome of the starting circumstances. If it is natural, how does that differ from a naturalistic explanation? Surely all you are doing is invoking some external agency, which is an unnecessary philosophical/theological step."
To which my answer would be: Riiiiiiiiiight. So I toss a coin and it comes up heads, That's luck. I do it again. Heads - luck again. I do it a hundred times, all heads. Luck. I shuffle a pack of cards and deal them - all the spades in order, then all the hearts in order, then all the diamonds in order, then all the clubs in order. Luck. I press keys at random on a computer, and come up with the complete works of Shakespeare. Luck.
What Behe is saying is, yes, you could possibly contrive a series of circumstances in which events happen in such a way that we end up where we are today. But to describe this as the outcome of "random" mutations, and the design of the universe as being "lucky" would be disingenuous. If things are that specific - tailored that carefully to produce life - then darwinism is certainly not the right picture, because this is not a "random" outcome - it is highly non-random, just as the circumstances that led to the moon being the size it is must have been highly specific, and the fine tuning of fundamental constants is also highly non-random. As has been pointed out before, once a sufficiently low level of probability is reached (Ford and Arthur turning into penguins?), we basically say, "Impossible." I don't believe that it is reasonable to say that something is so improbable that it is (to all accounts and purposes) impossible, but then shrug and say, "Well, it happened. We're here, aren't we?"
Let's put it another way. Suppose the world in which we find ourselves does turn out to be an incredibly low-probability outcome. In these circumstances, what would falsify the darwinian view of the world? If you can say that, no matter how improbable the outcome, it was just random mutations getting lucky, then under what circumstances can you say that darwinism isn't true? If there are none, then darwinism is every bit as unfalsifiable as it is claimed ID is by its opponents.
Behe also looks at the multiverse idea - does this have more explanatory power? He argues not - and specifically, if you have an infinite number of universes, he argues, then you have explained nothing at all, since in an infinite number of those infinite number, the entire universe will in fact be a trick of our consciousness, rather than something that really exists.
I have my reservations about Behe's thoughts here - that everything might be regarded as a "natural" outcome. My hunch - though I don't have the science to back it - is that if Christianity is true, there has been "supernatural" input at various stages - perhaps corresponding with the six biblical days of creation. However, this is theology, not science ....
