It seems teleology—seeing in nature Aristotle’s final cause—is impossible not to assume, no matter how hard you try.
Just look at this Wikipedia article on the concept of exaptation, which it defines as “a shift in the function of a trait during evolution.”
The article states that the term exaptation was coined specifically to avoid associations with teleology. But in the opening paragraphs, before that claim is even made, the writer gives as an example of exaptation the idea that bird feathers may have evolved for temperature regulation but then were adapted for flight (emphases mine):
Bird feathers are a classic example. Initially they may have evolved for temperature regulation, but later were adapted for flight. When feathers were first used to aid in flight, that was an exaptive use. They have since then been shaped by natural selection to improve flight, so in their current state they are best regarded as adaptations for flight. So it is with many structures that initially took on a function as exaptations, once molded for that new function they become adapted for that function.
https://en.m.wikipedia.org/wiki/Exaptation
The irony that evidently flew over the writer’s head (pun definitely intended) is that to say something exists for a purpose is just another way of saying it has a telos. Teleology is the idea that everything has an end external to itself and toward which it is oriented as it goes about doing what it does.
Why is this a problem for science? Because to say that something is there for an external reason or purpose is just to assume that there is something there for that thing to be for.
And not just there metaphorically but there in reality as a purpose or fuction. Without such an external end toward which a thing is oriented, it’s not clear how the existence of the thing in its natural context is even intelligible.
Take the example of a valve in your heart. We could accurately describe it as a flap of muscle and stop there. But that would miss the purpose of the flap: to keep blood flowing in one particular direction as the chambers alternate between low and high pressure. To describe the flap without reference to this purpose is precisely to exclude what makes it not just a flap but a valve.
In other words, a heart valve is only intelligible if we consider its telos.
But if this is true, then we have to say that the telos is ontologically prior to the thing. Without the valve-ness we just have a flap of muscle. And explaining how the heart works is impossible without reference to the real, exsiting purpose the flap of muscle fulfills as a valve.
And if the purpose is prior to the thing that “takes it on” or embodies or fulfills it, then we must ask where the purpose came from. Did the universe generate these purposes through a naturalistic process? Note that this question does not assume that valveness must exist apart from minds and flaps of muscles in hearts. It only observes that the universe has things existing in it that are unintelligible without valvensess and so must have been joined with the form of valveness, which implies that valveness is prior to any particular valve identifiable by science.
Which leaves us with the question: where did valveness come from? Can the physical sciences tell us without recourse to teleology?
Much is at stake in these questions. If science admits that teleology is necessary, then it must admit the existence of universals and other immaterial objects as real features of the world. And if it does that, then it’s game on.
Can there be purpose without design? Can there be design without intent? Can there be intent without mind? You see where all of this can lead, and why advocates of scientism understand that teleology is the hill upon which they are willing to die.
So science faces a conundrum. If the particular things that science takes as its objects of inquiry are only intelligible with reference to something like Aristotle’s fourth cause, then science must at the very least admit that the strict materialism it adheres to is a methodological contrivance. A productive contrivance, perhaps, but a contrivance nonetheless.
Science also must admit that it doesn’t show us the deepest reality, but only reality in the context of causes that it must assume but that it excludes from its analysis as a matter of methodology.
How do the defenders of methdological naturalism respond to the problem of teleology?
Some argue that the use of language that sounds like teleology in scientific discourse is only metaphorical. When Stephen Jay Gould refers to “selfish genes,” he isn’t literally arguing that the telos of genes is self-replication. He is just simplifying things for his audience.
Others recognize there is a real problem here. They argue that, ideally, natural science would rid itself of reliance on such metaphors. Appeals to teleology constitute a kind of linguistic training wheels that science no longer needs now that it has disproven the existence of, or at least rid itself of the need for as a mater of methodology, final causes.
Well I say, let’s hear it. I want to know what hard-core, “real” science sounds like without reference to teleology. I want to know all about how things came to operate the way they do without any purpose or function. And remember, no implicit teleological assumptions, like saying what something is for or talking about the functional role it plays within a system. Because that would be assuming a telos for that thing.
I want to hear exaptation defined without invoking the concept of function.
If science cannot escape the teleological language of the functions and purposes and external ends toward which the things it studies are oriented, then why should we believe that science has gotten rid of teleology?
If a theory can’t articulate itself, perhaps there is something wrong with the theory.
(For a more in-depth and highly entertaining exploration of this topic, see chapter 6 of Edward Feser’s The Last Superstition).