Examine the basic claims of the two ideas (intelligent design and Darwinism) with regard to a particular biochemical system like, say, the bacterial flagellum. 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.
To level the playing field,] one needs to relax Darwin’s criterion from this:
If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
to something like this:
If a complex organ exists which seems very unlikely to have been produced by numerous, successive, slight modifications, and if no experiments have shown that it or comparable structures can be so produced, then maybe we are barking up the wrong tree. So, Let’s break some rules!
— Michael Behe, from Science and Evidence for Design in the Universe, p. 145-147