This will be the obligatory anti-intelligent design post. The first two points I want to make are not original, but the third I don't recall seeing anywhere.
Point the first.
- Intelligent design is neither a scientific theory nor a scientific concept.
I'll take it as definitional that a set of scientific propositions that fail to explain anything at all are not any sort of a theory. So, insofar as intelligent design does not explain, it is not a scientific theory. And that intelligent design does not explain is almost self-evident. Upon confronted with the mysterious "box" containing all the biological processes and assorted "stuff" that produced biological creatures like us, evolutionary biologists try to suss out what's inside the box; while, on the other hand, intelligent design "theorists" put the word "GOD" on the box and call it a day. Naming is not explaining (I trust that is obvious!). Hence, on the first horn, intelligent design is not science.
Of course, even if intelligent design is just a name, it might be part of a theoretical apparatus that gave it explanatory power. Let me illustrate with an example: fibromyalgia syndrome. We might then construct a putatively scientific proposition involving it: "Anyone who exhibits more than 12 of 15 tender points has fibromyalgia syndrome." This, in itself, does not offer an explanation of why the tender points constitute a pathology. And, it seems that no explanation is forthcoming. That doesn't, in itself, refute (in the strong sense) the claim that fibromyalgia syndrome is be part of science, but it is very good reason to doubt it -- it shifts the burden of proof onto the defender of FMS. I can't, off the top of my head, think of a scientific concept that, if it does not make up a theory in itself, is not backed up or supported by a structured theory.
So, if intelligent design is supposed to be a concept, and not a theory, it must have a theoretical apparatus to give it explanatory power. I don't know what that apparatus could look like. If biological reality is a result of the intelligent works of a more powerful being, then the only way we could lend explanatory power to the concept is to find that being. This is analogous to the case of a watch -- with apologies to Paley. If I find a watch, and I call it "watch", this concept "watch" cannot be used to explain anything -- such as why the watch tells the time -- unless I either know how the watch works, or I find the person who made it (and who can thus assure me that it will work). Note that I'm setting the bar lower in the case of biological life than in the case of the watch -- I'm only suggesting that we need to find the intelligent designer, not that we need to be able to ask for and receive reassurance from the designer that things will work as we take them to.
Since intelligent design "theorists" don't have God in their back pockets, it follows that there is no theoretical apparatus that backs up the explanatory power of the concept. Hence, it is not science on this horn, either.
Point the second.
- There seems to be a residual hatred of expertise driving the intelligent design movement -- a claim that biology is very easy and no one really needs training in it before they can be qualified to speak on it. (Note that nothing I'm saying here constitutes a biological claim -- this is all purely philosophical.)
This is bad enough for the laity, but reprehensible for the academically-trained defenders, and, for that matter, those who are qualified as priests and the like. The academics would never take seriously an attack on the founding principles of their own disciplines by an uneducated outsider; furthermore, the priests would never listen to a biologist's words (as expertise) on the Gospels. And the laity, in their own special areas of expertise -- the filing system in their offices, the precise combination of key strokes to slaughter end-bosses in Halo 2, or what have you -- would also not tolerate non-experts trying to declare themselves knowledgeable. Hence, the motivation behind intelligent design is likely incoherent.
Finally, point the third.
- There is a philosophical confusion underlying intelligent design -- a presumption of metaphysical or ontological naturalism, as opposed to methodological naturalism.
Now, that's really all that follows from just the naturalistic explanation of biological reality. In order to conclude more, and justify claims about other orders of reality -- say, the mental/intentional or the moral -- we have to make reductionist or eliminativist or identity claims between the levels. That is, for example, we'd have to claim that the mental is just the physical, or the moral is just the socially beneficial (and the socially beneficial is just what leads to reproductive advantage). In short, we'd have to make a stronger commitment than methodological naturalism, a commitment that looks a lot like a kind of (limited) ontological naturalism.
Intelligent design "theorists" don't seem to question ontological naturalism, however; there's a thought running underneath the view that, if the naturalistic explanation of biology works, then the mental and the moral are entirely off the table as God's provinces. But, as I've just argued, that won't follow from methodological naturalism -- it'll only follow from the much stronger, and much more controversial, ontological naturalism. Now, ontological naturalism may indeed be true (I doubt it, but I have been wrong before), but its truth will not be established (although it might be suggested) by the presence of naturalistic explanations in biology. For intelligent design proponents to think otherwise is for them to accept ontological naturalism.