A class is - according to classical theory (Aristotle) - a collection of entities or elements sharing a common attribute or property. The process of assigning elements to classes is termed classification.


According to classical theory are all members of a class sharing some necessary and sufficient properties, which define membership of the class. According to the idea of ‘family resemblance’ (Wittgenstein) is this not necessarily the case. A family resemblance is a vague grouping of things that are together because of their similarities, but have no one thing in common. The example Wittgenstein uses is that of games; all games do not have one underlying trait in common, but are grouped together under the heading ‘games’.

"There is, in any case, a difference in how we talk about types and kinds versus how we talk about classes. We say that Socrates is a token of a type, or an instance of the natural kind, human being. But notice that we say instead that Socrates is a member of the class of human beings. We would not say that Socrates is a "member" of the type or kind, human beings. Nor would we say he is a type (or kind) of a class. He is a token (instance) of the type (kind). So the linguistic difference is: types (or kinds) have tokens (or instances); classes, on the other hand, have members." (Wikipedia, 2006c).


The prefix super- indicates a rank above, the prefix sub- indicates a rank below. In biology the prefix infra- indicates a rank below sub-. For instance:
  • Superclass
  • Class
  • Subclass
  • Infraclass

The concept of "class" is related to the concept of "concept": A concept defines a category or class of individuals. For example, the concept [animal] defines the classes of entities such as cats, dogs, fish, etc. 


In a classification system like Dewey Decimal Classification may a class (e.g., 780 Music) be expressed by a notation and by a verbal phrase. It may also be described by a note indicating the definition of the class, what to be included and what to be excluded. Such a note corresponds to the intensional definition of a concept (what one must know in order to identify the referent of the concept, i.e., the entities which belong to the class). By enumerating a list of all concepts classified in the particular class is given an extensional definition of that class. Some library classifications, such as DK5 provide special indexes enumerating all concepts identified with a given class. An alternative is to retrieve a set of documents classified by a given class and look at the terms used in the titles of the classified documents. This method is often used of classifiers in order to decide where to classify a document.


Concepts and classes are defined or used on the basis of some conceptualization of the domain, which is being classified. The most important issue in classification is the the rationale behind given definitions and conceptualizations of classes.







Wikipedia, the free encyclopedia. (2006a). Class. Disambiguation page.


Wikipedia, the free encyclopedia. (2006b). Class (Computer science).


Wikipedia, the free encyclopedia. (2006c). Class (philosophy).


Wikipedia, the free encyclopedia. (2006d). Class (set theory).



See also: Concept in Knowledge Organization;



Birger Hjørland

Last edited: 26-02-2007