Here at Public Discourse, Adam Seagrave (see his original post here) has been discussing evolution with Stephen Barr and Kenneth Kemp. In his latest essay, Seagrave argues that, since Darwinian evolution makes the various species of living things not real and immutable features of objective reality but merely useful ways for human beings to organize and categorize that reality, Darwin imperils the foundations of human knowledge.
In particular, Seagrave relies on the Aristotelian doctrine that human knowledge is ultimately founded on a process of induction or abstraction from sense experience, “a process that allows equal access on the part of the layman and specialist alike.” From this common experience, we come to know “stable and defined universal concepts,” including the various species of living things, which “form the building blocks of all subsequent knowledge.” Hence, “the very way in which we human beings know the world around us … is inextricably bound up with the stability and distinctness of the types of things, or ‘species,’ we observe.” Because evolutionary theory “carries with it a decisive rejection of the truth of this stability and distinctness of species or types of things,” it undermines the foundations of human knowledge and entails “a repudiation of our fundamental intellectual affinity with the world around us.”
I think quite a lot has gone wrong here. Regardless of how we come to have universal concepts, and assuming for the sake of argument that things have essences in the way Aristotle thought, it is clear that the concepts of our common experience do not generally correspond with the essences or species of things. For example, it is certainly essential to elephants that they have DNA, are composed of cells, and circulate their blood, but we learn these things from laborious scientific investigation, not from studying the concept elephant or from casually observing elephants. Similarly, it is essential to gold that the atoms of which it is composed have 79 protons in their nuclei, but people had a concept of gold for millennia before we learned this fact from atomic theory.
Moreover, sometimes our common experience positively misleads us about essences. A man who regularly sees caterpillars and regularly sees butterflies but never witnesses the process of metamorphosis would think that these were animals of two different species, not one. And, to take a famous example from the history of science, for centuries everyone thought that “jade” named a single kind of mineral until, in 1863, Alexis Demour discovered that passing under the single name “jade” were two chemically distinct substances—nephrite and jadeite—that are indistinguishable by casual observation. Again, our concepts do not match up with the essences of things.
Nor does this divergence between our concepts and essences depend on modern scientific examples. On the contrary, it was well-understood in the Aristotelian tradition. Thus, Thomas Aquinas, as a good Aristotelian, often said that we do not generally know the essences of things, once remarking that a philosopher could spend a lifetime studying a fly and not achieve a full knowledge of its nature.
But, if our common experience does not disclose to us the essences of things, how do we successfully interact with the world and communicate with each other? How do we recognize elephants when we see them and talk about elephants intelligently if we don’t know the essences of elephants? The answer is that there is a crucial difference between knowing the meaning of the word elephant and knowing the essence of the things that are elephants. (Hence, scholastic philosophy distinguished between nominal definitions, which tell us the meanings of words, and real definitions, which tell us the essences of things.) Competent speakers of the language know the meaning of the word, and this is what is required to recognize elephants when we see them and to talk about them intelligently. The second, our knowledge of the essences of elephants, remains very much a work in progress—a work carried out primarily by biologists and other scientists.
Hence, I cannot agree with Seagrave when he says that “we human beings experience the world in terms of stable and defined universal concepts.” We experience the world through concepts, to be sure, but they are not stable and they are seldom well-defined. Rather, our concepts evolve as our knowledge improves. In the seventeenth century, we had a concept of phlogiston, but we now know there never was any such thing. Understanding lightning, which man had experienced since his first days on earth, required the invention in the eighteenth century of new concepts—especially those of positive and negative charge—that we owe to Franklin. For millennia, the stable and defined concepts of Euclidean geometry governed our understanding of the physical world—until about a hundred years ago, when we learned from Einstein that the geometry of our universe is not Euclidean after all, a fact that radically alters our understanding of every physical thing.
So concepts come and concepts go. Whatever may be our current set of concepts, there is no guarantee that they mirror the structure of reality. They represent, rather, our current best guess about what the world is like. We may discover something tomorrow that upsets our current concepts, causing us to discard some, change others, and invent new ones.
More generally, only the most naïve thinkers have imagined that, for every concept we have, there is an essence in reality. I dig three holes in a field; is each a thing in the world sharing the essence of holeness? More strikingly, Aquinas thought that human artifacts such as tables and chairs have no essences: they are mere “accidental wholes” composed of various natural substances (which, for Aquinas, did have essences) arranged in a certain way. For him, therefore, our concepts related to artifacts do not correspond to essences of things in the world. We thus have many different kinds of concepts, which relate to reality in various and complex ways, and even when our concepts seem to refer to types or species of things, there is no guarantee that the world is the way our concepts imply.
This brings us back to Darwin. It may well be true that, prior to Darwin, our commonsense understanding divided living things into distinct, stable species, and it is certainly true that evolutionary theory contradicts this view. But this in no way imperils the foundations of knowledge. Anyone who had been paying attention would have already known that our concepts do not generally reflect the essences of things, including with respect to living things. Yes, if Darwin is right, we need to adjust some of our concepts, the biggest adjustment being that the borders between species turn out to be not sharp but blurred, just as the borders between various colors are not sharp but blurred. To be sure, this is a big change (which is one reason Darwin was a great scientist), but it is just one more improvement in our understanding of reality on a par with other scientific discoveries that have caused us to adjust our understanding of reality—our concepts—in significant ways.
Finally, there is a larger point here, and it concerns the sociology of knowledge. Darwin’s theory is a scientific theory about how living things and their various features change over time. An immense amount of empirical evidence tends to show that this theory is true. Like all scientific theories, Darwin’s could be refuted tomorrow by new evidence, but at this point that seems extremely unlikely. It would thus be astonishing if, as Seagrave would have it, there is some simple argument, whether philosophical or otherwise, not in any way involving new evidence, that disproves that theory at a stroke. Just based on the common experience of mankind, I’m much more willing to believe that a few thousand genetic mutations can turn an archaeopteryx into an eagle than that an obscure passage from Aristotle’s Posterior Analytics can refute evolution.