I can’t speak for David Hart, Joe, but I don’t think he was expressing (to use your words) “opposition to considering [the] possibility” of “Intelligent Design.” I think he was saying that the ID arguments lack the kind of rigor that some ID people seem to imagine they have.
I agree with you that there is nothing wrong with making “arguments from personal incredulity.” All of us necessarily make judgments based on what we find reasonable or plausible or antecedently probable. And you are right in observing that both ID and anti-ID people make such judgments. Some ID people think natural explanations of certain complex biological structures are as unlikely as walking to the moon. And some anti-ID people think miraculous explanations of these same structures are as unlikely as walking to the moon.
Still, I think Hart is basically right.
One can interpret “irreducible complexity” arguments in two ways. (a) They could be seen as attempts at rigorous proofs that certain structures, such as the blood clotting system or the bacterial flagellum, could not have arisen by small successive steps. In your words, they could be seen as attempts to “rule out” that possibility. Or (b), they could be regarded merely as statements that it is difficult to see how certain things could have arisen by small steps and that it is therefore, as you put it, “warranted [to search] for an alternate explanation.” As (a), irreducible complexity arguments fail. If one is to take them at all seriously, therefore, one must regard them as (b). But to regard them as (b) is to say, in effect, that they are arguments from personal incredulity. That is not to damn them. (And I don’t think Hart intended to damn them.) It is only to say that they fall very far short of being proofs of anything.
Why do I say they fall far short of being proofs? Consider an example that several people have used: a Roman arch. A Roman arch cannot stand unless all the stones that compose it are in place. And that would suggest that it cannot be built one stone at a time—i.e. it would seem to be “irreducibly complex”. Nevertheless, there is a way it can be built up one step at a time: First build a stone wall one stone at a time and one layer at a time. Then remove stones, one at a time, to leave an arch. This illustrates the difficulty in demonstrating the irreducible complexity of something. And that was Hart’s point: irreducible complexity “can never be logically demonstrated.” Or, at least, it is hard to carry out such a demonstration in practice.
Another point I think worth emphasizing: If an ID-skeptic says that a miraculous explanation of the complexity of certain structures seems to him as unlikely as someone having walked to the moon, it is not necessarily because he is “committed to atheistic materialism.” For example, I believe that miracles, by God’s power, can and occasionally do occur. Therefore, I believe that God could miraculously transport someone to the moon. Nevertheless, I think it improbable in the highest degree that anyone has ever been miraculously transported to the moon or ever shall be. In the same way, and for much the same reasons, a person who is a believer both in God and in the possibility of miracles might judge it highly implausible that complex biological structures such as the blood clotting system or the bacterial flagellum arose by miracles—which, in effect, is what ID claims.
Finally, while it perfectly reasonable to put forward the possibility that certain biological structures arose by miracles, that does not mean that the ID conjecture is a scientific one and should be taught as such in science classes. As I have said elsewhere, Moses parted the Red Sea, but that was not a new effect in hydrodynamics; miraculous cures may occur at Lourdes, but they are not new discoveries in oncology; and Jesus turned water into wine, but he did not thereby teach us anything new about organic chemistry.
When I read Mike Behe’s fascinating book Darwin’s Black Box, I was impressed at the complexity of the biological structures he described. They do indeed give the appearance of irreducible complexity. And this constitutes a great challenge for biological science. (That word “challenge”, by the way, is in the title of his book.) But the challenge does not seem to me to be manifestly impossible, like walking to the moon by natural means. But, then, that is the thing about personal incredulity, isn’t it? What is credible to one person is not credible to everyone.