"Intelligent design advocates, however, exploit the ambiguity between process and product that is built into the word 'design.' For them, the presence of a finished product (a fully evolved eye, for instance) is evidence of an intelligent design process. But this tempting conclusion is just what evolutionary biology has shown to be mistaken."For example:
1. The eye appears to have a purpose, a design.
2. What bears a design - houses, cars, paintings - must have been begotten by a designer.
3. Therefore, eyes were designed by an unknown designer.
In the first line, "design" is used to name a quality of the eye as a finished product, but in the second line, design is used in the sense of "begotten by a designer," a process. So a semantic shift has occured, thus rendering the argument fallacious.
Darwin, as usual, spoke of the complexity of the eye most eloquently, under the heading "Organs of Extreme Perfection and Complication":
"To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real."