§6.2, “The What and Why of Categories” explained what categories are and the contrasting cultural, individual, and institutional contexts and purposes for which categories are created. In doing so, a number of different principles for creating categories were mentioned, mostly in passing.
We now take a systematic look at principles for creating categories, including: enumeration, single properties, multiple properties and hierarchy, family resemblance, similarity, and theory- and goal-based categorization.
The simplest principle for creating a category is enumeration; any resource in a finite or countable set can be deemed a category member by that fact alone. This principle is also known as extensional definition, and the members of the set are called the extension. Many institutional categories are defined by enumeration as a set of possible or legal values, like the 50 United States or the ISO currency codes (ISO 4217).
Enumerative categories enable membership to be unambiguously determined because a value like state name or currency code is either a member of the category or it is not. But there comes a size when enumerative definition is impractical or inefficient, and the category either must be sub-divided or be given a definition based on principles other than enumeration.