ABSTRACT
Essentialists believe that there are objective, mind-independent, kinds of things in nature. These are the so-called “natural kinds.” To explain the existence of these natural kinds, essentialists postulate that the sources of the relevant similarities and differences are intrinsic, i.e. independent of circumstances, and independent of human knowledge or understanding. Things of the same natural kind are supposed to have certain intrinsic properties or structures that together explain their manifest similarities, whereas things of different natural kinds are supposed to be intrinsically different in ways that adequately account for their manifest differences. The properties or structures that distinguish the kinds are called their “real essences.” The real essences of natural kinds are to be distinguished from their nominal essences. The real essence of a kind is the set of properties or powers that a thing must have for it to be a thing of that kind. The nominal essence of a kind (whether natural or not) is the set of properties or powers that a thing must have, or perhaps just the set of predicates that must be satisfied, for it to be called a thing of that kind. In either case, the statement attributing the essence to the kind is necessarily true; for there is no possible world in which it would be false. But the two kinds of necessity are nevertheless different. The kind of necessity that is associated with real essences is metaphysical, or de re, necessity, while that associated with nominal essences is analytic, or de dicto. The difference lies not in the strength of the necessity that is attributed to the relationship, but in its grounding. De re necessities are grounded in the real world, and have to be discovered by scientific investigation. Specifically, we have to discover what sets of intrinsic properties or structures are required to constitute things of these kinds. De dicto necessities are grounded in our linguistic conventions, and can be discovered by competent speakers of the language just by reflecting on how the terms designating the kinds are used. De re necessities are thus a posteriori and need to be established empirically, whereas de dicto necessities are knowable a priori. Natural kinds may be supposed to exist in many different fields of inquiry. Accordingly, we may distinguish between essentialists by their commitments to
natural kinds. To be an essentialist in biology, for example, is to believe that there are natural biological kinds, each of which has its own distinctive real essence. To be an essentialist in chemistry is to believe that there are natural chemical kinds having real essences. To be an essentialist in ontology is to believe that at least some of the most fundamental existents in nature are members of natural kinds, and that things of these kinds are distinguished by their own real essences. Aristotle was a biological essentialist. He believed that animal species were natural kinds that were distinguished from one another by their essential natures. Hilary Putnam is a chemical essentialist, as his Twin Earth example illustrates. But most of us who would claim to be essentialists without qualification are ontological essentialists. That is, we believe that natural-kinds structures go all the way down to the most basic levels of existence. This does not mean that we believe that these same sorts of structures exist at all higher levels. In fact, very few essentialists these days would claim to be economic or even biological essentialists. Most would accept chemical essentialism, the case for which appears to be overwhelming, and some form of physical essentialism, but would be skeptical of essentialist claims about the existence of natural kinds at higher levels of complexity. Every distinct type of chemical substance would appear to be an example of a natural kind, since the known kinds of chemical substances all exist independently of human knowledge and understanding, and the distinctions between them are all real and absolute. Of course, we could not have discovered the differences between the kinds of chemical substances without a lot of scientific investigation. But these differences were not invented by us, or chosen pragmatically to impose order on an otherwise amorphous mass of data. There is no continuous spectrum of chemical variety that we had somehow to categorize. The chemical world is just not like that. On the contrary, it gives every appearance of being a world made up of substances of chemically discrete kinds, each with its own distinctive chemical properties. To suppose otherwise is to make nonsense of the whole history of chemistry since Lavoisier. What is true of the chemical kinds is not true of biological species. The existing species of animals and plants are clusters of morphologically similar organisms whose similarities are due to their genetically similar constitutions. Our species concepts are therefore generic cluster concepts. They are not, however, generic kinds that are categorically distinct from one another, as the generic chemical kinds are. The species “elephant” has a number of sub-species, which are sub-clusters within the elephant cluster. These sub-species are distinct enough to be reliably distinguished morphologically, and sufficiently different genetically to be said to be different kinds of animals. However, if we broadened our vision to include all of the ancestors of the current elephants in the world, we should find, I think, that the morphological clusters, and the genetic clusters that explain them, would shift about as we go back in time, and would eventually overlap. Therefore, neither the generic species nor any sub-species of elephant is a natural kind in the same sense as the generic and specific chemical kinds are. Chlorine, for example, is a generic chemical kind, the species of which include the various isotopes of chlorine. But there is no species of chlorine existing now or at any other time that could possibly be a species of any element other than chlorine.
