The Semantics and Metaphysics of Natural Kinds - CRC Press Book
Philosophers question whether the multiple divisions used by biologists reflect an ontological pluralism in the world or, alternatively, whether there is a privileged conception of species that captures ontological reality. Furthermore, some philosophers argue that species are not to be considered as natural kinds at all, insisting instead that species are individuals.
Despite its long history and intuitive appeal, the conception of species as natural kinds is difficult to sustain while also maintaining a traditional view of what a natural kind requires: a set of intrinsic natural properties that are individually necessary and jointly sufficient for a particular to be a member of the kind. The fact that lineages evolve more or less gradually over time and that this process leads to new species and other taxa, has two consequences: first, that species are spatio-temporally restricted in the sense that the species to which a particular organism belongs depends on its being related to a specific lineage; and secondly, that the characteristic properties of a species may change over time.
The first consequence of evolution implies that, contra the traditional view, intrinsically identical organisms may not be members of the same species: a cat-like organism independently evolved on a distant planet would not be a cat. The second consequence implies that intrinsic similarity is not necessary for membership of the same species.
There exists a high degree of variation in intraspecific morphology and genetic makeup. While members of kinds need not be intrinsically identical e.
That, it is claimed, is absent for species. Thus there is no genetic material or sequence of genes that all and only members of the species Drosophila melanogaster possess, and likewise for all other species. Nor can we turn to larger-scale phenotypic properties which may nonetheless be hidden , since evolutionary change may eliminate such features without a new species arising Sober Furthermore, gradual change, even through speciation, means that species will not be categorically distinct criterion 6 , which for Ellis is an additional reason to conclude that species are not natural kinds.
These problems for the thesis that species are natural kinds may lead one to conclude that classification by species does not correspond to any real division of things in nature any more than the higher taxa do. Darwin himself expressed this kind of conventionalism, in taking species to differ only in degree from varieties on the one hand and genera and higher taxa on the other hand:. Mishler contains a modern version of this view. The rejection of species as natural kinds need not lead to the rejection of realism about species altogether. Indeed, one can commit to species realism in a specific way: Ghiselin , , , , Hull , , , , and many other philosophers and biologists accept the claim that species are individuals , not kinds.
This claim would appear to explain why, it is said, there are no serious candidates for biological laws, at least concerning members of particular species Beatty Individual organisms are parts of species, not members of the species-kind.
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Speciation creates a new individual or possibly two new individuals and the cessation of the preceding species-individual but not a new kind. Organisms that are parts of the same species may share common features, but that is not what makes them parts of that species. Rather the explanation is the reverse: because the organisms are parts of the same species they are parts of the same lineage and so they will probably but not inevitably share features in common.
Does individualism about species imply that species are not natural kinds? One might suspect as much, but perhaps the two views are compatible. LaPorte , for example, believes that one can consistently hold both views.
Where one theorist will see a species-as-kind and an organism-as-member of that kind, another theorist will see a species-as-individual and an organism-as-part of that kind. For example, an individual organism a radish plant is a part of the species-individual Raphanus Sativus if and only if it is a member of the species-kind Raphanus Sativus LaPorte , For that reason, it is also less plausible that kinds must play a role in laws of nature.
If one retains the view that species are natural kinds one must confront the fact that a plethora of species concepts are available and as seen earlier, these species concepts do not always delineate the same species. These definitions are not perfectly coextensive. For a detailed discussion of species concepts see the entry on species. This diversity may be thought to support conventionalism about natural kinds Section 1. But supporters of naturalism about our classifications Section 1. A monist will hold that one of the existing systems is a superior account of the natural world.
The Semantics and Metaphysics of Natural Kinds
A commitment to one of the different systems over another will result in a different ontological account of species. The most prevalent system of classification in contemporary systematics is cladistics, which classifies organisms phylogenetically, in virtue of shared derived features or an organism's place in the genealogical tree.
An alternative view to monism, motivated by the variety of species concepts, is pluralism , which holds that the different accounts of species in systematics reflect equally legitimate ways of carving up nature subject to our pragmatic and theoretical interests. However, these systems do carve nature in genuinely natural ways. The species and higher taxa that they delineate reflect real features of the world from the point of view of different theoretical interests. Kripke , and Putnam a use animal kinds as examples of natural kinds for which a posteriori essences can be found.
There is some implication that these essences are microstructural, intrinsic properties, which will be, of necessity, individually necessary and jointly sufficient for an entity to be a member of a kind. However, if species are individuals, then it is not true that species may be individuated on the basis of the intrinsic properties of their members. The various species concepts tend to offer relational criteria of species membership see above. According to the BSC, for example, membership of a species depends on relational properties, such as membership of a certain population and interbreeding.
Alternatively, the PSC refers to shared descent. However, Kripke himself argues that a person's parentage is essential to them. If that is so, then if individual S is descended from N , then S is necessarily descended from N. McGinn suggests that this extends to species also. LaPorte argues that essentialism holds with respect to facts relating individuals, species, and other taxa to the higher taxa genus and above within which they are nested.
These taxa are clades, that is to say kinds defined by shared descent from a common ancestral group: an individual or group that is a member or part of clade is necessarily a member or part of that clade. Thus biological kinds species, genera, etc. So one option for biological essentialism is to drop the traditional view that the natural properties that are essential for membership of a natural kind are intrinsic: natural, extrinsic relational properties can play this role Okasha The categorical distinctness criterion must be dropped also.
On account of the problems for natural kinds in biology as well as the development of modern chemistry, chemical kinds have replaced biological kinds as the paradigms of natural kinds. The chemical elements and chemical compounds appear to be bona fide natural kinds.
We refer to chemical kinds in laws, explanations, and inductions: that a certain item is iron explains its behaviour and that behaviour is predictable; that iron objects are magnetizable is a law of nature criteria 2 and 3 above. Moreover, we can induce that all iron is magnetizable, from the observation of particular instances of iron objects that are magnetic. And chemical kinds appear to obey the categorical distinctness requirement: iron is clearly distinct from its neighbours in the periodic table manganese and cobalt ; no elements are intermediate kinds criterion 6 above.
Furthermore, microstructural essentialism seems to be a prima facie plausible option for chemical kinds: it is essential to iron that something made of pure iron is constituted by atoms that have precisely 26 protons in their nuclei. Microstructuralism in the philosophy of chemistry is the thesis that chemical kinds can be individuated solely in terms of their microstructural properties Hendry As exemplified above by the case of iron, the chemical elements provide paradigmatic kinds that may be individuated microstructurally, since the atomic number—the number of protons in the nucleus—suffices to identify the element.
It is true that macro-level chemical and physical properties can also serve to individuate chemical elements, and in the nineteenth century chemists were able to individuate elements without knowing what we do about nuclear structure. However, it is held that atomic number has explanatory priority: the number of protons in the nucleus, and hence the nuclear charge, explains the structure of electrons surrounding the nucleus, which, in turn, explains the chemical behaviour of the element.
The microstructuralist can extend this approach from elements to compounds. Compounds are identified principally by their constituent elements. Thus carbon dioxide is that compound of carbon and oxygen with the molar proportion In more detail, molecules of carbon dioxide consist of two oxygen atoms and a single carbon atom.
The practice of identifying a chemical compound only by its composing elements was the norm in chemistry, until the discovery of isomerism by Friedrich Woehler in Isomers are distinct compounds that nonetheless share the same constituent elements in the same proportion. Thus fulminic acid and cyanic acid may both be expressed in terms of constituents in the empirical formula CHNO, but their distinct chemical and physical properties identify them as different substances.
Thus molecules of isomers have the same atoms in different spatial arrangements, e. Isomerism means that specifying the chemical composition alone is not sufficient for classification. The microstructure of a compound concerns not just the elemental atoms in its molecules, but also their spatial arrangement. Needham and van Brakel have argued that compounds such as water are dynamic structures whose natures cannot be given in static accounts of their composing elements. Molecules of H 2 O are polar, with the consequence that the electropositive hydrogen atoms in one H 2 O molecule will bond with the electronegative oxygen atoms in another H 2 O molecule, such bonds being hydrogen bonds with the result that in liquid water H 2 O molecules will form polymer-like chains known as oligomers.
The hydrogen bonds and the consequent chains are responsible for the fact that water is liquid at room temperature whereas compounds with similarly sized molecules, such as hydrogen sulphide, methane, and carbon dioxide, are all gaseous at room temperature. These oligomers are constantly forming, breaking, and reforming. The rate of such changes and the mean length of such oligomers are dependent on the thermodynamic context in the extreme case, an ice crystal may be considered as a single such oligomer, the crystal structure being dependent on the fixed, strong hydrogen bonds.
Thus, say Needham and van Brakel, we cannot consider water to be just a compound composed of a collection of H 2 O molecules. Furthermore, anything that is to be water must be a macroscopic entity, since only macroscopic bodies can bear thermodynamic properties, such as melting point, which we use to identify water.