Classification and categorization: a difference that makes a difference

Library Trends, Wntr, 2004 by Elin K. Jacob

Proposition II states that, because each member of the category must exhibit all of the essential features that comprise the intension of the category, possession of the set of features that defines the category is sufficient to determine membership in the category. And, because there is a binary, either/or relationship that exists between an entity and a category such that an entity either is or is not a member of a particular category, the boundaries of categories are said to be fixed and rigid.

Proposition III identifies the inheritance relationship that exists between categories in a hierarchical structure: any member of a category that is a subset of a superordinate category must exhibit not only the set of essential features that determine membership in the subset but also the set of essential features that determine membership in any superordinate category within which the subset is nested.

In its most rudimentary form, categorization can be defined as the placement of entities in groups whose members bear some similarity to each other. Within the framework of the classical theory of categories, however, categorization is the process of systematically dividing up the world of experience into a formalized and potentially hierarchical structure of categories, each of which is defined by a unique set of essential feature(s). Because the intension of a category defines the set of essential features that each member of the category must exhibit, the classical theory maintains that intension equals extension--that membership within a particular category (extension) entails possession of the essential and defining character (intension) of the category. For example, if the intension of the category "bird" consists of the features "lays eggs," "has wings," "flies," and "builds nests in high places," every member of the category must exemplify the complete set of defining features. If an entity does not fly, it cannot be accorded membership in the category "bird" even if it does lay eggs, have wings, and build nests in high places. And, because all members of the category are defined by the same set of features, no one bird can be more typical or more representative of the category than any other bird. Thus, according to the classical theory, a parrot, a pigeon, and a puffin would be equally representative of the category "bird."

Brown (1979) observes that within the formalized and rigidly constrained ordering of reality established by the classical theory, category membership is absolute: "... any given thing is either in or out of the set" (p. 189). It is this stipulation that is the source of the classical theory's explanatory power: because it requires that intension equals extension--that membership in a category demonstrates possession of the set of essential features that define the category--the classical theory of categories would provide a simple yet elegant explanation for both the internal structure of cognitive representations and the semantic meanings of words.