A theist friend requests a design argument. Here is one.
You are out hiking and the trail becomes faint and hard to follow. You peer into the distance and see three stacked rocks. Looking a bit farther, you see another such stack. Now you are confident which way the trail goes.
Your confidence is based on your taking the rock piles as more than merely natural formations. You take them as providing information about the trail's direction, which is to say that you to take them as trail markers, as meaning something, as about something distinct from themselves, as exhibiting intentionality, to use the philosopher's term of art. The intentionality, of course, is derivative rather than original. It is not part of your presupposition that the cairns of themselves mean anything. Obviously they don't. But it is part of your presupposition that the cairns are physical embodiments of the original or intrinsic intentionality of a trail blazer or trail maintainer. Thus the presupposition that you make when you take the rock piles as providing information about the direction of the trail is that an intelligent being designed the objects in question with a definite purpose, namely, to indicate the trail's direction.
Of course, the two rock piles might have come into existence via purely natural causes: a rainstorm might have dislodged some rocks with gravity plus other purely material factors accounting for their placement. And their placement might be exactly right. Highly unlikely, but possible. This possibility shows that the appearance of design does not entail design. A stack of rocks may appear to be a cairn without being one. A cairn, by definition, is a marker or memorial, and thus an embodiment of meaning, meaning it cannot possess intrinsically in virtue of its mere physicality, e.g., its being a collocation of bits of rhyolite.
Nevertheless, your taking of the rock piles as trail markers presupposes (entails) your belief that they were put there by someone to mark the trail. It would clearly be irrational to take the piles as evidence of the trail's direction while at the same time maintaining that their formation was purely accidental. And if you later found out that they had come into being by chance due to an earthquake, say, you would cease interpreting them as meaning anything, as providing information about the trail. One must either take the rock piles as meaningful and thus designed or as undesigned and hence meaningless. One cannot take them as both undesigned and meaningful. For their meaning -- 'the trail goes that-a-way' -- derives from a designer whose original intentionality is embodied in them.
In short: the rock stacks have no meaning in themselves. They have meaning only as embodying the original intentionality of someone who put them there for a purpose: to show the trail's direction. The hiker who interprets the stacks as meaningful presupposes that they are embodiments or physical expressions of original intentionality and not accidental collocations of matter.
Now consider our incredibly complex sense organs and brain. We rely on them to provide information about the physical world. I rely on eyesight, for example, both to know that there is a trail and to discern some of its properties. I rely on hearing to inform me of the presence of a rattlesnake. I rely on my brain to draw inferences from what I see and hear, inferences that purport to be true of states of affairs external to my body. The visual apparatus (eye, optic nerves, visual cortex and all the rest) exhibits apparent design. It is as if the eyes were designed for the purpose of seeing. As we say colloquially, eyes are for seeing. But the appearance of design is no proof of real design. And indeed, human beings with their sensory apparatus are supposed to have evolved by an unguided process of natural selection operating upon random mutations. If so, eye and brain are cosmic accidents. The same goes for the rest of our cognitive apparatus: memory, introspection, reason, etc.
But if this is the case, how can we rely on our senses to inform us about the physical world? If eye and brain are cosmic accidents, then we can no more rely on them to inform us about the physical world than we can rely on an accidental collocation of rocks to inform us about the direction of a trail.
As a matter of fact, we do rely on our senses. Our reliance may be mistaken in particular cases as when a bent stick appears as a snake. But in general our reliance on our senses for information about the world seems justified. Our senses thus seem reliable: they tend to produce true beliefs more often than not when functioning properly in their appropriate environments. We rely on our senses in mundane matters but also when we do science, and in particular when we do evolutionary biology. The problem is: How is our reliance on our sense organs justified if they are the accidental and undesigned products of natural selection operating upon random mutations?
To put it in terms of rationality: How could it be rational to rely on our sense organs (and our cognitive apparatus generally) if evolutionary biology under its naturalistic (Dawkins, Dennett, et al.) interpretation provides a complete account of this cognitive apparatus? How could it be rational to affirm both that our cognitive faculties are reliable, AND that they are accidental products of blind evolutionary processes? That would be like affirming both that the cairns are reliable trail indicators AND that they came about by unguided natural processes. I agree with Richard Taylor who writes:
. . . it would be irrational for one to say both that his sensory
and cognitive faculties had a natural, nonpurposeful origin and
also that they reveal some truth with respect to something other
than themselves, something that is not merely inferred from them.
(Metaphysics, 3rd ed. p. 104)
This train of thought suggests the following aporetic triad or antilogism:
1. It is rational to rely on our cognitive faculties to provide access to truths external to them.
2. It would not be rational to rely on our cognitive faculties if they had come about by an unguided process of natural selection operating upon random genetic mutations.
3. Our cognitive faculties did come about by an unguided process of natural selection operating upon random genetic mutations.
The limbs of the triad are individually plausible but collectively inconsistent: they cannot all be true. From any two limbs one can validly argue to the negation of the remaining one. So, corresponding to our antilogism there are three valid syllogisms. One of them is a design argument that argues to the negation of (3) and the affirmative conclusion that behind the evolutionary process is intelligent, providential guidance. "And this all men call God."
To resist this design argument, the naturalist must reject either (1) or (2). To reject (2) is to accept the rationality of believing both that our cognitive faculties arose by accident and that they produce reliable beliefs. It is to accept the rationality of something that, on the face of it, is irrational. To reject (1) is not very palatable either. But I suppose one could bite the bullet and say, "Look, we are not justified in relying on our cognitive faculties, we just rely on them and so far so good."
A mysterian naturalist could say this: Our cognitive faculties came about through an unguided evolutionary process; it is rational to rely upon them; but our cognitive architecture is such that we simply cannot understand how it could be rational to rely on processes having this origin. For us, the problem is insoluble, a mystery, due to our irremediable limitations. Just because it is unintelligible to us how something could be the case, it does not follow that it is not the case.
The best objection to this little design argument I have sketched comes from the camp of Thomas Nagel. Nagel could say, "You have given good reason to reject unguided evolution, but why can't the guidance be immanent? Why must there be a transcendent intelligent being who supervises the proceedings? Nature herself is immanently intelligible and unfolds according to her own immanent teleology. You cannot infer theism since you haven't excluded the pansychist option."
Of course, one could beef up the design argument presented by working to exclude the panpsychist option.