Classification and categorization: a difference that makes a difference

Library Trends, Wntr, 2004 by Elin K. Jacob

Taxonomic classification supports the efficient storage and retrieval of information about a class of entities, but reliance on a systematic approach such as that advocated by Linneaus constrains the information context by limiting the identification of knowledge-bearing associations to hierarchical relationships between classes. Furthermore, class definitions based on a single feature such as reproductive structure effectively reduce the amount of meaningful information that can be represented about each class in the taxonomy.

Classification Schemes.

A classification scheme is a set of mutually exclusive and nonoverlapping classes arranged within a hierarchical structure and reflecting a predetermined ordering of reality. Because a classification scheme mandates that an entity can be a member of one and only one class, it provides for communication of meaningful information through the systematic and principled ordering of classes. Furthermore, it establishes and enforces stability of reference by providing each class with a unique label that links individual members of the class to the class definition.

Shera (1951/1065) observes that, throughout history, attempts to classify knowledge have relied on four basic assumptions: universal order, unity of knowledge, similarity of class members, and intrinsic essence. The assumption of universal order posits an immutable conception of reality that serves as a unifying framework for all knowledge. The assumption of unity of knowledge presupposes that past, present, and future knowledge can be represented within a single, inclusive hierarchy of superordinate and subordinate classes. The assumption of similarity of class members holds that a class can be defined by a set of essential features and that these features are shared by all members of the class and distinguish that class from all other classes in the structure. And the assumption of intrinsic essence maintains that there is a set of individually necessary and jointly sufficient features that is intrinsic to all members of a class and that these features constitute the essence of the class.

With the possible exception of universal order, Shera's exposition of the assumptions that support efforts to organize knowledge can be interpreted in terms of the three propositions that constitute the classical theory of categories: the assertion that a category is defined by a summary representation (Proposition I) is a statement of the essential similarity of class members; the assertion that a category is defined by a set of essential features (Proposition II) is a statement of the intrinsic essence of a class; and the assertion that defining features are inherited in a hierarchical structure of categories (Proposition III) is a statement of the unity of all knowledge. It is instructive that, although the classical theory of categories is unable to account for the variability and flexibility of cognitive categorization, it does provide an elegant accounting of the fundamental assumptions on which classification schemes have historically been constructed.