Thomas Nagel on Inferring Either Way"Public Education and Intelligent Design" in Philosophy and Public Affairs 36, no. 2 (Wiley Periodicals: 2008), pp. 188-9.
From the beginning it has been commonplace to present the theory of evolution by random mutation and natural selection as an alternative to intentional design as an explanation of the functional organization of living organisms. The evidence for the theory is supposed to be evidence for the absence of purpose in the causation of the development of life-forms on this planet. It is not just the theory that life evolved over billions of years, and that all species are descended from a common ancestor. Its defining element is the claim that all this happened as the result of the appearance of random and purposeless mutations in the genetic material followed by natural selection due to the resulting heritable variations in reproductive fitness. It displaces design by proposing an alternative. ¶ No one suggests that the theory is not science, even though the historical process it describes cannot be directly observed, but must be inferred from currently available data. It is therefore puzzling that the denial of this inference, i.e., the claim that the evidence offered for the theory does not support the kind of explanation it proposes, and that the purposive alternative has not been displaced, should be dismissed as not science. The contention seems to be that, although science can demonstrate the falsehood of the design hypothesis, no evidence against that demonstration can be regarded as scientific support for the hypothesis. Only the falsehood, and not the truth, of ID can count as a scientific claim. Something about the nature of the conclusion, that it involves the purposes of a supernatural being, rules it out as science.