Thanks to Darwin, we now know that Paley was wrong. We can explain the complex functionality of organisms without positing a god. Still, Darwinians have to admit that Paley was half-right: This complex functionality does demand an explanation. In fact, most evolutionary biologists would affirm some of Paley's language: Yes, animals were "designed;" it's just that the "designer" was natural selection, not God.
Of course, natural selection doesn't work like a watchmaker. It doesn't think ahead and create new features that will add functionality to an organism. Rather, it creates new features randomly, blindly, and then the dysfunctional ones get weeded out as the organisms possessing them die young or for some other reason fail to reproduce. Richard Dawkins, alluding to Paley, called natural selection "the blind watchmaker" in a book by that name. But a blind watchmaker is still a watchmaker. Organisms do have a designer, even if the designer is a somewhat clumsy process, not a conscious, far-seeing intelligence.
Dennett has long accepted Dawkins's line of thought, and he has long accepted one extension of it: that natural selection has imbued organisms with "goals," with "purpose". Specifically: the goal of organisms is to get genes into subsequent generations. That may not be their conscious goal, but it is nonetheless the basic thing they were "designed" to do. (And their other apparent "goals" are subordinate to it. All animals seek food, for example, but that goal was itself favored by natural selection only because it helped animals survive long enough to transmit their genes.)
In short: Dennett has long believed that William Paley was right to look at organisms and surmise that (a) they had a designer (in some sense of the word); and (b) this designer had imbued them with goals, with an overarching purpose (however ignoble a purpose genetic proliferation may seem to us).
The gist of the argument I made to Dennett was this: What if you took this part of Paley's logic-the valid part-and applied it not to individual organisms, but rather to the whole system of life on this planet? Doesn't it suggest that the whole system had a designer (again, in some sense of that word). To see what I mean, let's look again at an organism through Paley's eyes, only this time let's look at its whole life span, starting at the very beginning.
A single egg cell replicates itself, and the offspring cells in turn replicate themselves, and so on. Eventually the resulting lineages of cells start exhibiting distinctive specialties; there are muscle cells that beget muscle cells, brain cells that beget brain cells. If Paley were around today to watch videos of this process he would say: Wow!-Look at how exquisitely directional this process is; the system grows in size and in functional differentiation until it becomes this large, complex, functionally integrated system: muscles, brains, lungs, etc. This directionality is evidence of design!
As it happens, you can describe the history of evolution on this planet in a way that closely parallels this description of an organism's life cycle. First, a few billion years ago, a single primitive cell divides. The resulting offspring cells in turn replicate themselves, and eventually different lineages of cells (that is, different species) emerge. Some of these lineages eventually become multicellular (jellyfish, birds) and exhibit distinctive specialties (floating, flying, etc.).
Meanwhile, as the human species is becoming a global brain, gradually assuming conscious control of the planet's stewardship, other species-also descended from that single primitive cell that lived billions of years ago-perform other planetary functions. Trees are lungs, for example, generating oxygen.
In other words: If you watched evolution on this planet unfold from a distance (and on fast forward), you would find it strikingly like watching the maturation of an organism ("epigenesis"). So why can't the part of Paley's argument that can be validly applied to an organism's maturation-the idea that it suggests a designer of some sort-be applied to the whole system of life on earth?