Natural kinds

3. Essentialism

Locke (1689) and, following him, Kripke (1972) and Putnam (1975), each offer an account of how our understanding of a given natural kind can develop over time. At an initial stage, speakers classify certain objects as members of the same natural kind because they perceive those objects to have certain properties in common. Such common properties form the ‘nominal essence’ (Locke) or ‘stereotype’ (Putnam) associated with that kind. The speakers, however, further believe that these objects are members of the same natural kind if and only if they have some underlying, though perhaps currently unknown, common property. This is the ‘real essence’ of that kind. In the case of chemical elements, it will perhaps be some microstructural property. In the case of organisms, it will perhaps be some genetic structure. Moreover, given how laws of nature relate a kind’s real essence to its nominal essence, an object’s real essence explains why that object has the nominal essence that it does. It then falls to empirical science to discover the real essences of objects.

In sum, an object’s nominal essence serves as a heuristic, a guide for thinking whether that object is a member of a certain natural kind. But whether in fact that object is a member of the kind in question is determined by whether it has the appropriate real essence. For instance, various objects may once have been classified as members of the natural kind lemon because of their yellow appearance, tart taste, thick skin and oval shape. These properties form the kind’s nominal essence. However, scientific inquiry may subsequently reveal the real essence of lemons – a common genetic structure, let us suppose. Since something is a lemon if and only if it has this genetic structure, we may revise to some degree our original classification of which objects are lemons. On this view, fruit which are superficially similar to lemons but which have a different genetic structure, are not lemons. In contrast, unripe, mouldy, or squashed fruit with the same genetic structure as lemons are lemons, even though superficially they do not appear to be so.

Kripke and Putnam further argue that it is (metaphysically) necessary that something is a member of a given kind if and only if it has a certain real essence. Hence if the real essence of, for example, gold is a certain atomic number, then, necessarily, something is gold if and only if it has that atomic number. Kripke and Putnam conclude that there are certain de re necessities which can be discovered only empirically (see De re/de dicto; Essentialism). Other philosophers have since argued that essentialism about natural kinds entails that the laws of nature which govern kinds are also de re necessary.