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

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Abstract
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.

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