Classification and categorization: a difference that makes a difference

Library Trends, Wntr, 2004 by Elin K. Jacob

Taxonomic Classification.

Classification is perhaps best exemplified by the discipline of taxonomy. Broadly defined, taxonomy is the science of classification or, as Mayr (1982) defines it, "the theory and practice of delimiting kinds of organisms" (p. 146). The objectives of taxonomic investigation are to provide an orderly and systematic organization of knowledge about the biological world; to identify the defining characteristics that distinguish a biological entity; and, based on those characteristics, to place the entity within a hierarchical ordering of mutually exclusive superordinate and subordinate classes in accordance with a set of established and widely accepted principles.

Taxonomic classification establishes stability of nomenclature through the aegis of a formalized and universally accepted language that facilitates transmission of knowledge across time and the barriers of natural language. Each class in the taxonomic scheme is given a unique name that is used to refer to all entities that display the complete set of features defining the class. And, because it is universally employed to identify all members of a given class, this label provides access to the accumulated knowledge about those entities, not as individuals but as members of a particular class. The taxonomic name establishes a relationship of equivalence between the set of features that define the class (its intension) and the set of entities that are members of the class (its extension). Using the taxonomic name, a member of a biological class is recognizable wherever it occurs, regardless of natural language or the local name (s) by which it may be known.

Through the inheritance of definitional criteria made possible by enforcing a principled structure of superordinate and subordinate classes, taxonomic classification also serves as an external cognitive scaffolding (Clark, 1997; Jacob 2001, 2002) that provides for the economical storage and retrieval of information about a class of entities. For example, the observation that Bleu is a poodle provides information about Bleu that is associated with the class "poodle." More importantly, however, it also provides information about Bleu that is available from the hierarchical structure within which the class "poodle" is nested--information associated with the superordinate classes dog, mammal, vertebrate, etc.

The essential observation, however, is that the practice of taxonomy is carried out within the arbitrary framework established by a set of universal principles. For example, while the naturalist Adanson, a contemporary of Linneaus, proposed a method for organizing botanical phenomena based on the identification of differences between individual specimens (Foucault, 1970), Linneaus advocated a systematic approach based on similarity of reproductive structure. For the naturalist following Linneaus's lead, any physical differences between two specimens not directly related to the process of reproduction would be irrelevant: for example, differences of leaf, stem, or root structure that might be used to distinguish between two plants would be ignored if the plants exhibited similar reproductive structures.