Literature

Two criticisms of ID are commonly heard: (1) it is unfalsifiable; and (2) it is falsified by observations of imperfect adaptations. Taking the second objection first, philosopher Elliott Sober acknowledges that there is a valid response: 'How can anyone know what an intelligent designer wanted to achieve unless the designer tells us?' Those who continue to use this style of criticism would do well to digest Sober's comments.
However, the first criticism is, in Sober's judgment, more substantial. Nevertheless, Popper's criterion of falsifiability should not be invoked if the grounds are that design inferences are probabilistic. If Popper's criterion is used in this way, not only would ID be deemed unfalsifiable, but also much evolutionary theory! Consequently, Sober invokes the necessity of considering "auxiliary propositions" in evaluating ID's scientific claims. These propositions, in other contexts, allow theories to make specific predictions. In the context of ID, "we have no independent evidence concerning which auxiliary propositions about the putative designer's goals and abilities are true". Thus, the robust response to the second criticism (noted above) is actually a demonstration that the first criticism is valid.
Sober's paper certainly needs to be discussed within ID circles. Suffice to say here that his reformulated criticism interacts only with a small subset of possible ID enquiries that purport to explain why an observed design was selected from the range of options. This is not a substantial criticism because ID scholars are concerned with making design inferences, not identifying design goals.
Sober's criticism needs to focus more on the mainstream ID arguments relating to objective evidences of design and the recognition of irreducible complexity. The probabilistic recognition of design is an integral part of archaeological science and forensic science, and any philosophy of science that does not find a place for design inferences must be defective. Furthermore, it is not a convincing argument against irreducible complexity to talk about genetic drift or crossing valleys in the fitness landscape. What we need, on the part of critics of ID, is a more penetrating analysis of actual ID arguments.

WHAT IS WRONG WITH INTELLIGENT DESIGN?
Elliott Sober
The Quarterly Review of Biology, March 2007, vol. 82, pp. 3-8.

ABSTRACT: This article reviews two standard criticisms of creationism/intelligent design (ID): it is unfalsifiable, and it is refuted by the many imperfect adaptations found in nature. Problems with both criticisms are discussed. A conception of testability is described that avoids the defects in Karl Popper's falsifiability criterion. Although ID comes in multiple forms, which call for different criticisms, it emerges that ID fails to constitute a serious alternative to evolutionary theory.

See also:

What is Wrong with Sober's Attack on ID? Evolution News, Posted by Casey Luskin
(Part I): Defining ID and its Historical Origins (March 21, 2007)
(Part II): Comparing ID and Darwinism while Ignoring Darwinism's Epicycles (March 28, 2007)
(Part III): Ignoring the Widely Discussed Positive Predictions of Intelligent Design (March 30, 2007)
(Part IV): Sober's Regressive Arguments (March 31, 2007)