Replacement of an Essentialistic Perspective on Taxonomic Definitions as Exemplified by the Definition of "mammalia"

•Current controversies about the definitions of taxon names reflect different underlying philosophical perspectives concerning the nature of definitions. The antithetical perspectives, called methodological essentialism and methodological nominalism (Popper, 1966, The open society and its enemies, Princeton Univ. Press, Princeton, NJ), are exemplified by alternative views in a controversy surrounding the definition of the name "Mammalia." Traditional perspectives on the definition of "Mammalia" are essentialistic in that the definition takes the form of a description stated in terms of the traits of individual organisms, thus implying that taxa are abstract categories, i.e., that taxa have essences. In addition, the extension of the defined term (the set of species or organisms to which the name applies) is logically prior to its intension or defining formula (the property or properties that a species or organism must have to be designated by the name). Consequently, the name is treated as if it had a designation proper to it, which conforms with the essentialistic view that the name is an abbreviated description of the essence. An alternative perspective is manifested in the redefinition of "Mammalia" as the name of the monotreme and therian crown clade. This perspective contrasts with the traditional one in that the definition is stated in terms of common ancestry relationships rather than organismal traits, thus implying that taxa are concrete composite wholes rather than abstract categories. In addition, the defining formula is logically prior to the extension of the defined term, and thus the name is treated only as a convenient label or shorthand symbol for the defining formula that has no proper designation. The replacement of an essentialistic perspective on the definitions of "Mammalia" and other taxon names by a more nominalistic one is associated with the development of a phylogenetic perspective on biological nomenclature and represents an important step in the development of a more broadly scientific approach to that subject. [Definition; essentialism; nominalism; Mammalia; nomenclature; philosophy; taxonomy; Karl R. Popper.] It has been said that "in no other science is definition as important as it is in taxonomy" (Hull, 1965:315). Recently, the importance of definitions in taxonomy has been highlighted by the realization that explicitly phylogenetic methods for defining the names of taxa are central to the development of a phylogenetic system of biological nomenclature (de Queiroz and Gauthier, 1990, 1992, 1994). But many of the taxon names in current use, particularly the oldest and most familiar, were coined long before the advent of phylogenetic definitions. Indeed, many were coined before widespread acceptance of the concept of phylogeny itself. Consequently, it should not be surprising that definitions formulated in the context of the phylogenetic system are coming into conflict with older definitions of the same names. An example of this conflict concerns the name "Mammalia," which has been defined phylogenetically as the crown clade stemming from the most recent common ancestor of monotremes and therians (Gauthier et al., 1988a, 1988b, 1988c, 1989; Rowe, 1988; de Queiroz and Gauthier, 1992). This definition has been contested by systematists adopting a view that has become traditional among paleontologists, according to which "Mammalia" designates a taxon including not only the monotreme and therian crown clade but also several of its extinct outgroups (Miao, 1991; Lucas, 1992; for further discussion see Rowe and Gauthier, 1992; Bryant, 1994). On one level, the disagreement is nothing more than a difference in preferences concerning the association of a taxon name with one clade or another. Here, I argue that it also reflects