I have taken this from a press release on Eurekalert, issued by the University of Chicago - it was so interesting I am posting the whole release.
In a thought-provoking paper from the March issue of The Quarterly Review of Biology , Elliott Sober (University of Wisconsin) clearly discusses the problems with two standard criticisms of intelligent design: that it is unfalsifiable and that the many imperfect adaptations found in nature refute the hypothesis of intelligent design.
Biologists from Charles Darwin to Stephen Jay Gould have advanced this second type of argument. Stephen Jay Gould's well-known example of a trait of this type is the panda's thumb. If a truly intelligent designer were responsible for the panda, Gould argues, it would have provided a more useful tool than the stubby proto-thumb that pandas use to laboriously strip bamboo in order to eat it.
ID proponents have a ready reply to this objection. We do not know whether an intelligent designer intended for pandas to be able to efficiently strip bamboo. The "no designer worth his salt" argument assumes the designer would want pandas to have better eating implements, but the objection has no justification for this assumption. In addition, Sober points out, this criticism of ID also concedes that creationism is testable.
A second common criticism of ID is that it is untestable. To develop this point, scientists often turn to the philosopher Karl Popper's idea of falsifiability. According to Popper, a scientific statement must allow the possibility of an observation that would disprove it. For example, the statement "all swans are white" is falsifiable, since observing even one swan that isn't white would disprove it. Sober points out that this criterion entails that many ID statements are falsifiable; for example, the statement that an intelligent designer created the vertebrate eye entails that vertebrates have eyes, which is an observation.
This leads Sober to jettison the concept of falsifiability and to provide a different account of testability. "If ID is to be tested," he says, "it must be tested against one or more competing hypotheses." If the ID claim about the vertebrate eye is to be tested against the hypothesis that the vertebrate eye evolved by Darwinian processes, the question is whether there is an observation that can discriminate between the two. The observation that vertebrates have eyes cannot do this.
Sober also points out that criticism of a competing theory, such as evolution, is not in-and-of-itself a test of ID. Proponents of ID must construct a theory that makes its own predictions in order for the theory to be testable. To contend that evolutionary processes cannot produce "irreducibly complex" adaptations merely changes the subject, Sober argues.
"When scientific theories compete with each other, the usual pattern is that independently attested auxiliary propositions allow the theories to make predictions that disagree with each other," Sober writes. "No such auxiliary propositions allow … ID to do this."
In developing this idea, Sober makes use of ideas that the French philosopher Pierre Duhem developed in connection with physical theories – theories usually do not, all by themselves, make testable predictions. Rather, they do so only when supplemented with auxiliary information. For example, the laws of optics do not, by themselves, predict when eclipses will occur; they do so when independently justified claims about the positions of the earth, moon, and sun are taken into account.
Similarly, ID claims make predictions when they are supplemented by auxiliary claims. The problem is that these auxiliary assumptions about the putative designer's goals and abilities are not independently justified. Surprisingly, this is a point that several ID proponents concede.