In @pnelson’s windscreen note example (which I accept is not the best one might find), the significant point is that the principle sign of “intelligent agency” (like, a person) is not that a paper is under the windscreen with squiggles on it like examples we’ve seen before, but that we can discern final causation through the whole phenomenon - note and squiggles in the context, and most of all the message that tells us the guy’s purpose, or final cause, in putting it there.
Paul is right about the immaterial nature of that purpose, as in the content of the posts on the thread he mentions. For the most part science cannot deal in final causes, and they, like formal causes, were excluded from it at Bacon’s time as being useless for doing science, because they are subjective.
They’ve crept back into biology in the shape of “biological functions”, and even there prove their “tricksyness” by often fooling us: the “sexually preferred” antlers of the giant elk turn out to be a result of a structural law in deer (Gould); the giraffe turns out not to use its long neck to reach the higher branches at all.
To put that in Paul’s context, there is nothing in the material or efficient causes leading to the note on the windscreen, or this post, that entails they should have that particular purpose: the note could be a hoax, or the posts put here for some cynical reason other than the conversation we assume. This post might contain a clever code to alert a lurker to plant a bomb.
In other words, even in human affairs final causation is to a degree intuited, and not objectively reproducible. Yet we can’t do without it, and we do a pretty good job of discerning it as purpose, even when we don’t understand that purpose. Indeed, usually in our consideration it completely replaces the scientific business of efficient causation - Paul is scarcely interested whether his note is on notepaper or a bus ticket, in pencil or fountain pen. And none of us here, I’ll wager, gives a thought to whether each other is typing on a tablet or PC, or dictating into voice recognition software. I go to get a coffee without even thinking what muscles to use. The mind connects directly with final purpose and takes the rest for granted.
Not so for God? Well, at least equally risky, which is why finality of all kinds is usually outside science. But we routinely discern final causation in biological function, and whilst giant elk may fool us, we know eyes are for seeing, fins for swimming and feedback loops for homeostasis. Mostly, we’re right.
We discern that function by the same intuition which is the only tool we have for discerning final causation in human activity, and the question is not so much whether it belongs in science or not by being “natural”: the same final cause could be the result of aliens, angels, God or my own mind playing tricks on me. No, the question is whether our direct mind-finality connection is correct (as with windscreen notes and forum posts), or whether our faculty has been fooled by purely efficient causes (chance and necessity) giving the illusion of finality.
In that case, “the eye is for seeing” is merely my folk-psychological way of saying, “following this sequence of causes, the eye turns out to see.” “Normal serum potassium” really means “within the range that a majority of similar organisms have except when they don’t” and so on. It gets complicated without finality!