The Idea of Species and the False Story about Essentialism

A major objection to my notion of Darwinian natural right is that Aristotelian natural right assumes an essentialist understanding of species that has been refuted by Darwinian evolutionary science.

My idea of natural right assumes that human beings exist as a distinct species or kind of animal with characteristic traits, which include natural desires that incline human beings to certain natural ends. I can then argue that whatever fulfills those ends of the human species constitutes the natural human good.

But Darwin denied the eternal fixity of species by claiming that species emerge by a historically contingent process of evolution from ancestral species, which would seem to deny the objective reality of species boundaries, since they are in perpetual flux. According to biologists like Ernst Mayr and philosophers like David Hull, Darwin's intellectual revolution was his denial of the ancient Platonic and Aristotelian conception of species as defined by eternal, discrete, and unchanging essences, and his affirmation of species as nothing more than historical lineages that come into being and pass away. Hull has concluded from this Darwinian denial of essentialism that since Homo sapiens is a historically contingent entity, like every other species, there is no such thing as human nature. David Buller, a student of Hull's, has elaborated this point in arguing that evolutionary psychologists do not understand how the evolutionary account of species as historical lineages denies any belief in human nature as anything more than a "superstition."

In chapter 9 of Darwinian Natural Right, I responded to this line of thought by arguing that although the Darwinian must deny the eternity and fixity of species, and thus deny the claims of a transcendentalist essentialism, the Darwinian must still affirm the reality of species as natural kinds. Although species are not eternally fixed, since they have evolved from ancestral species, that does not make them any less real during the time of their existence. I also argued that Aristotle in his biological works anticipated the modern Darwinian criticisms of essentialist classification. Aristotle's biology was misinterpreted in the Middle Ages by those religious believers who viewed his logical concept of "species" as a biological concept of fixed kinds conforming to the teaching of biblical creationism, in which species become eternal ideas in the mind of God.

In the essentialist tradition of biological classification, logical division was used to classify organisms into genera and species with definitions based on essential properties that are necessary and sufficient for defining each species. So, for example, the essentialist definition of human beings was that they were the rational animals. They belonged to the genus of animals, while being uniquely distinguished from other animals by their rationality. The essence of human nature was thus captured through the necessary and sufficient properties of rational animality, properties that were fixed, discrete, and unchanging.

But contrary to the common view, I argued, Aristotle did not employ this method in his biological writings. On the contrary, he criticized the artificiality of applying logical division to biological phenomena, as Plato did. In his logical works, Aristotle did define "species" through the possession of essential, or necessary and sufficient properties. But in his biological works, he generally accepted the popular classification of species and genera, while turning his primary attention to studying how living beings were adapted for a specific kind of life in a specific kind of environment. The essential traits of a biological species are essential because of the functional role they play in the life of the living being.

When Darwin claimed that all species have evolved from ancestral species so that each species is adapted to a specific manner of life, he was closer to Aristotle than to those nominalists who would deny the natural reality of species.

So, I am now pleased to report that the scholarly writing on the "species problem" seems to be moving towards this position as I argued it in 1998. Increasingly, historians of science and philosophers of biology are questioning the "essentialism story" told by Mayr and Hull that presents Darwin's "populational" thinking as a revolutionary rejection of the "essentialist" thinking that ruled over biology for two thousand years. Instead, scholars are rediscovering a tradition of Aristotelian biological empiricism that broke away from Platonic essentialism and prepared the way for Darwin.

Some of this new scholarship was surveyed a few years ago in an article criticizing the "essentialism story"--Mary Winsor, "Non-essentialist Methods in Pre-Darwinian Taxonomy," Biology and Philosophy, 18 (2003), pp. 387-400. Now we have two new books that elaborate the issues in this scholarly debate. Newly published is John S. Wilkins, Species: A History of the Idea (University of California Press, 2009). Soon to be published is Richard Richards, The Species Problem: A Philosophical Analysis (forthcoming from Cambridge University Press). I have read Wilkins' book, and I have read a few chapters from Richards' manuscript. Wilkins is a philosophy professor at the University of Sydney. Richards is a philosophy professor at the University of Alabama.

Wilkins provides an encyclopedic history of the idea of species from Plato to the present. Running through his history is his argument for the falsity of the essentialism story as told by Mayr and Hull. His argument rests on three claims (x-xi, 231-34).

His first claim is that the essentialism story fails to distinguish species as a logical concept from species as a biological concept. Living species are not the same as formal species. Beginning with Plato, there is a logical tradition of universal taxonomy, which attempts to classify all possible objects into categories defined by necessary and sufficient properties. Beginning with Aristotle, there is a biological tradition of reasoning about species that are identified by a range of traits that are not reducible to logical essences of necessary and sufficient properties.

Wilkins' second claim is that Aristotle's biological writings began a "generative conception of species" that runs through the tradition of natural history as empirical science. Living species are identified by their generative power, which constitutes a lineage by which living form is passed through a reproductive process. Sometimes Wilkins identifies this generative conception of species as beginning with Lucretius and the Epicureans (25-27, 227-228). But I would stress the importance of this generative conception for Aristotle in his book on the generation of animals.

Wilkins' third claim is that in the tradition of empirical biology, species are understood as types rather than essences, and types allow for variation, while essences do not. Biologists can recognize individual organisms as conforming to the type of a particular species, even though there is great individual variation around the type.

As I indicated in Darwinian Natural Right, Darwin is ambivalent about the reality of species. In some passages of his writing, he seems to be a nominalist or conventionalist who views the identification and classification of species as a matter of convenience for the human mind that has no ground in nature. But in other passages, Darwin affirms the natural reality of species by claiming that only evolutionary biology can uncover the natural basis of classification by seeing that "community of descent is the hidden bond which naturalists have been unconsciously seeking," and therefore, all true classification "must be strictly genealogical in order to be natural." Wilkins is not completely clear about this. Generally, he argues that Darwin believed in "species as real things in nature (albeit temporary things)"; and therefore he was a "species realist" (129-30). But then Wilkins also says that for Darwin "species are the outcomes of the evolutionary process acting on varieties and are not real entities themselves" (230).

The fundamental reason for this ambiguity, I think, is suggested by Wilkins' qualifying phrase--"albeit temporary things." In the Platonic tradition, there's a tendency to identify reality with eternity--what is really real must be eternal. That's why Platonists typically look to mathematics as a model of timeless truths. But unlike mathematics, biology is the study of temporal processes that are everchanging. Unlike mathematical objects, living things come into being and pass away. But Aristotelians and Darwinians can say that the temporality of living things and processes takes nothing away from their reality. A living being is real for as long as it exists.

Harvard biologist Louis Agassiz was one of the leading critics of Darwin's theory of evolution when Darwin proposed it in 1859. As a Christian Platonist, Agassiz regarded species as thoughts in the mind of God and therefore fixed essences that could not have evolved through any historical process. Here then is the Platonic essentialism that those like Mayr and Hull conjure up as the dominant tradition of biological thought prior to Darwin. For Agassiz, Darwin's denial of the eternal fixity of species was a denial of the very reality of species.

Agassiz wrote: "If species do not exist at all, as the supporters of the transmutation theory maintain, how can they vary? And if individuals alone exist, how can differences which may be observed among them prove the variability of species?" In a letter to Asa Gray in 1860, Darwin responded: "How absurd that logical quibble 'if species do not exist, how can they vary?' As if anyone doubted their temporary existence?" (quoted by Wilkins, 158).

Darwin did not doubt the "temporary existence" of species. But for a Platonic essentialist like Agassiz, "temporary existence" is not real existence at all.

Richard Richards goes more deeply than does Wilkins into the history of how Aristotle's biological studies of species were overlooked or distorted by a medieval tradition of Christian Platonism that read Aristotle as a Platonic essentialist who could be brought into alignment with a biblical creationist conception of species as eternally fixed in the mind of the Creator.

My defense of an Aristotelian and Darwinian conception of species and of the reality of human nature can be found in some previous posts here and here.