Grow Some TestablesIntelligent design ducks the rigors of science.

Four months ago, when evolution and "intelligent design" (ID) squared off in Kansas, I defended ID as a more evolved version of creationism. ID posits that complex systems in nature must have been designed by an intelligent agent. The crucial step forward is ID's concession that "observation, hypothesis testing, measurement, experimentation, logical argument and theory building"—not scriptural authority—define science. Having acknowledged that standard, advocates of ID must now demonstrate how hypotheses based on it can be tested by experiment or observation. Otherwise, ID isn't science.

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This week, ID is on trial again in Pennsylvania. And so far, its proponents aren't taking the experimental test they accepted in Kansas. They're ducking it.

The Pennsylvania case involves a policy, adopted by the board of the Dover Area School District, that requires ninth-grade biology teachers to tell students about ID. According to the policy, "A theory is defined as a well-tested explanation that unifies a broad range of observations." So far, so good.

Under the policy, "Students will be made aware of gaps/problems in Darwin's Theory and of other theories of evolution including, but not limited to, Intelligent Design." Notice the "of" before "other theories." The policy doesn't tell teachers to discuss gaps and problems in ID. It tells them to discuss gaps and problems in Darwinism—and then to discuss ID as an alternative "theory." The board's brief makes clear that the policy's aim is "informing students about the existing scientific controversy surrounding Darwin's Theory of Evolution, including the fact that there are alternative scientific theories."

The first half makes sense: Students should be made aware of gaps and problems in Darwinism. But what's with the second half? Once you've outlined the limits of Darwinism, what more does ID offer? What does it say? What does it explain?

So far, nothing.

The board names two scientists who advocate ID "as a scientific theory": Michael Behe of Lehigh University and Scott Minnich of the University of Idaho*. Minnich's expert testimony in the Dover case refers to Behe's work. Behe's testimony refers to a 2001 article in which he claims to have shown "that intelligent design theory is falsifiable." A longer version of the article explains,

In fact, intelligent design is open to direct experimental rebuttal. Here is a thought experiment that makes the point clear. In Darwin's Black Box (Behe 1996) I claimed that the bacterial flagellum was irreducibly complex and so required deliberate intelligent design. The flip side of this claim is that the flagellum can't be produced by natural selection acting on random mutation, or any other unintelligent process. To falsify such a claim, a scientist could go into the laboratory, place a bacterial species lacking a flagellum under some selective pressure (for mobility, say), grow it for ten thousand generations, and see if a flagellum—or any equally complex system—was produced. If that happened, my claims would be neatly disproven.

Behe is right that such an experiment, by showing that random mutation and natural selection can produce the flagellum, would disprove the claim that they can't. He calls the latter claim—that Darwinism fails to produce the flagellum—the "flip side" of his claim that the flagellum required intelligent design. But the Darwinism-fails claim isn't just the "flip side" of the design-is-necessary claim. It's the whole thing. The theory that's being tested in the experiment is Darwinism. If Darwinism succeeds, ID would be disproved, but only to the extent that ID consists of saying Darwinism would fail. And to that extent, ID isn't an explanatory theory in its own right. It's just a restatement of the first half of the Dover School Board's policy: a discussion of gaps in Darwinism.

Behe's article makes clear that ID is purely negative, with no explanatory mechanisms of its own.

The claim of intelligent design is that "No unintelligent process could produce this system." The claim of Darwinism is that "Some unintelligent process could produce this system." To falsify the first claim, one need only show that at least one unintelligent process could produce the system. To falsify the second claim, one would have to show the system could not have been formed by any of a potentially infinite number of possible unintelligent processes, which is effectively impossible to do.

The complaint that Darwinism can resort to an "infinite number" of processes misses the key word: processes. What makes Darwinism finite and falsifiable is its commitment to explain processes of evolution. Debunk one process, and Darwinists are forced to propose and test another. (For an excellent review of Darwinism's performance under empirical challenge, see Rick Weiss and David Brown's article in Monday's Washington Post.) What makes ID infinite and unfalsifiable is its refusal to explain intelligent design. You send your kids to biology class to learn by what processes living things evolve. ID doesn't even try to answer that question.

Don't take it from me. Take it from Behe. "By 'intelligent design' I mean to imply design beyond the laws of nature," he writes. Or take it from the Dover School Board, whose brief flatly denies "that Intelligent Design Theory sets forth a thesis concerning the nature of the intelligence responsible for the apparent design in nature." In his testimony, Behe even asserts that "the necessity for a 'scientific' theory to be falsifiable is disputed."

So here's what ID proponents are offering to teach your kids: They won't say how ID works. They won't say how it can be tested, apart from testing Darwinism and inferring that the alternative is ID. They won't concede it has to be falsifiable. All they'll say is that Darwinism hasn't explained some things. But that's what the first half of the Dover policy says already. So there's no need for the second half—the part that mentions ID.

The Dover School Board thinks it's getting a bum rap. All it asked its teachers to do was to mention ID. It never ordered them to teach it. "The theory of Intelligent Design shall not be taught to the students," says the board. Of course not. There's nothing to teach.

Correction, Sept. 30, 2005: The Dover school board submitted two briefs on July 13, 2005. One said Minnich was a professor at Iowa State. The other said he was a professor at the University of Idaho. Based on the first brief, I said he was at Iowa State. The second brief turns out to be the correct one. (Return to corrected sentence.)

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William Saletan is Slate's national correspondent and author of Bearing Right: How Conservatives Won the Abortion War.

Photograph of Earth on the Slate home page © Denis Scott/Corbis.

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