[Cf. an addendum here.]
Forms as functional essences. The myriad of particular "versions" of a seeing device produced by natural selection all are just that––variations on a theme. The formal essence of an "optical sensory device" transcends the various material instantiations of it known by biologists. The issue of formal causation versus sheer mechanism is very removed from the weight IDers might give the eye, or any other ornate biological apparatus, namely, that it is "too complex" not to be designed by an Intelligent Creator. The issue of form in nature has to do with the simple, twin ideas that the same formal operations can appear under many different material circumstances, and that if they could not, nature would be an unintelligible mishmash of material causal collisions. In other words, formal causality in nature not only orders material causation in organisms themselves but also orders our knowledge of material causation in objects of study.
It is precisely the formal operations in question which give both biological "value" to the organism's matter and theoretical clarity to the scientific inquiry. If there were not formal order in nature, we would not see anything distinct in nature, much as if there were no pattern "in" 3D illusions we would literally stare all day without seeing anything. Even if we were able to "see" patterns in nature, as an automatic confabulation process in our brains, we would have to admit the patterns we see are but illusions of order superimposed on nature by our precocious brains. Once you grasp the definition of an eye as, say, an organ for detecting light, you can immediately grasp that formal coherence in any eye you happen to find in nature. If one day a marine biologist found an unspecified critter in the coral with an unusual protrusion on its back, he could analyze the tissue all day without knowing what the protrusion is (much less what it is for). He might call it an "eye-like protuberance," but unless he understood the functional essence of the protrusion, he could not grasp what it is (nor, again, what it is for). If, however, he later discovered that a fine beam of light shone on the protrusion animated the critter, he would instantly realize the protrusion, innocuous in its material aspects, is an eye. By one stroke of metaphysical coherence, "the form of an eye" both activates the biologist's mind to grasp what the material protrusion is and orders that matter itself into being an actual eye. This is what Michael Polanyi, of whom more presently, meant by arguing the structure of our knowing really corresponds with the structure of the thing(s) known, otherwise our knowledge is falsely so-called.
It should go without saying that the formal (and final) discovery of the critter's eye dynamically includes the material and efficient modes of causality involved in constituting and "triggering" the eye. Indeed, it is the formal coherence of the protrusion which "promotes" merely "eye-like" matter on its back to the status of a genuine eye. This is not suggest that formal causality is mysteriously exempt from material or efficient causality. Far from it, which is why, first, Aristotle considered causation a unified reality albeit with a fourfold structure and, second, he rejected Platonic Forms in favor of saying that forms only exist as actual substances. On page 39 of The Tacit Dimension (New York: Anchor Books, 1966), Michael Polanyi expresses nicely the interrelations of formal and material causality by noting that
a complete physical and chemical topography of an object wold not tell us whether it is a machine, and if so, how it works, and for what purpose. Physical and chemical investigations of a machine are meaningless, unless undertaken with a bearing on the previously established operational principles of the machine. But there is an important feature of the machine which its operational principles do not reveal; they can never account for the failure and ultimate breakdown of the machine. … Only the physical-chemical structure of a machine can explain its failures. Liability to failure is, as it were, the price paid for embodying operational principles in a material the laws of which ignore these principles.
To take a second example: If you saw a lump of dissected organic tissue lying on the floor, nothing about its material structure would clearly indicate that it is an "eye." If, however, I told you I had dropped a fish eye on the floor, the lump of matter would instantly "become" an intelligible object––an eye––even despite the pitiful condition of the matter (having been plucked out of a fish's head and dropped on the floor). It is not because the tissue––the matter––was diseased or deficient that the lowly fish eye is no longer "an eye" qua an active seeing device; it is only because the matter, in itself perfectly sound, is no longer ordered by its formal nature that it is incorrect to call that chunk of organic tissue an eye.
Let me cite some of what Francis Beckwith recently posted about this, not the least because his post was the catalyst for this post of mine:
Although it is true that final causes imply design, the ID movement is a project in which the irreducible or degree of specified complexity of the parts in natural objects are [sic] offered as evidence that these entities are designed. But that is not the same as a final or formal cause, which is something intrinsic to the entity and not detectable by mere empirical observation. For example, if I were to claim that the human intellect’s final cause is to know because the human being’s formal cause is his nature of “rational animal,” I would not be making that claim based on the irreducible or degree of specified complexity of the brain’s parts. Rather, I would be making a claim about the proper end of a power possessed by the human person. That end cannot be strictly observed, since in-principle one can exhaustively describe the efficient and material causes of a person’s brain-function without recourse to its proper end or purpose. Yet, the end or purpose of the human intellect seems in fact to be knowable.
As Beckwith notes, and as I have intimated above, there is a close, perhaps even redundant, connection between formal and final causality. Indeed, to return to my earlier line of thought, the evolved success of versions of any formally coherent organ ("tool" in Greek) is evocative of the close connection between form and finality. The many versions of the eye in nature all "succeed" over time because they all accord with the proper function of an eye: namely to see better than not to see. As Stephen Barr writes:
Some people think that the Darwinian mechanism eliminates final causes in biology. It doesn't; the finality comes in but in a different way. Why does natural selection favor this mutation but not that one? Because this one makes the eye see better in some way, which serves the purpose of helping the creature find food or mates or avoid predators, which in turn serves the purpose of helping the animal to live and reproduce. Why do species that take up residence in caves gradually lose the ability to see? Because seeing serves no purpose for them, and so mutations that harm the faculty of sight are not selected against. … Darwinian explanations can account for very little indeed without bringing intrinsic finality into the explanation.