Classification and categorization: a difference that makes a difference

The acquisition and transmission of information are dependent not only on the cognitive ability to create new categories--and thus new information--through the discovery of new patterns of similarity across entities, but also on the ability to capture information about these patterns through the medium of language. With the accumulation of more specialized knowledge and the creation of disciplinary domains, however, these categories and the relations between them have a tendency to become formalized (Jacob, 1994). The need to ensure that disciplinary knowledge is consistent across individuals and across time privileges the stability of reference provided by well-defined classes. As experientially-based categories evolve into well-defined, domain-specific classes that facilitate sharing of knowledge without loss of information, they lose their original flexibility and plasticity as well as the ability to respond to new patterns of similarity.

THE CLASSICAL THEORY OF CATEGORIES

Until Rosch's publication in the 1970s of her seminal work on categories and categorization (Rosch, 1973, 1975), research in the area of categorization had focused on concept formation not as a process of creation but as a process of recognition. The world of experience was assumed to consist of a set of predetermined categories, each defined by a set of essential features represented by a category label; and all members of a given category were assumed to share a set of essential features that was identified by the category label and could be apprehended by all members of the linguistic community. Thus Hull (1920) wrote of the child's discovery of meaning in the word "dog" as the gradual recognition of a preexisting and invariant concept: "The 'dog' experiences appear at irregular intervals.... At length the time arrives when the child has a 'meaning' for the word dog. Upon examination this meaning is found to be actually a characteristic more or less common to all dogs and not common to cats, dolls and 'teddy-bears'" (Hull, 1920, pp. 5-6; cited in Brown, 1979, p. 188).

The presumption that a category is determined by a set of defining criteria is known as the "classical theory of categories." This is a simple but powerful theory that rests on three basic propositions (Smith & Medin, 1981; see also Taylor, 1989):

1. The intension of a category is a summary representation of an entire category of entities.

2. The essential features that comprise the intension of a category are individually necessary and jointly sufficient to determine membership within the category.

3. If a category (A) is nested within the superordinate category (B), the features that define category (B) are contained within the set of features that define category (A).

Proposition I states that the definition (intension) of a category is the union of the essential features that identify the membership (extension) of that category. Furthermore, because all members of a single category must share this set of essential features, each member is equally representative of the category as a whole. For this reason, the internal structure of a category is said to be ungraded, or without rank, because no member can be more typical or more representative of a category than any other member.