What is categorization or classification?


  • Classification: a partitioning (grouping) of items into classes with fixed, rigid boundaries.
  • Classes are defined by necessary and sufficient features; "necessary" means that the features apply to all items in a class; "sufficient" means that enough features are provided to distinguish the class from other classes.
  • Classification scheme: a hierarchy of classes. A class is a subclass of another class (superclass) if all the items of the subclass also belong to the superclass. The classes at the lower levels of the hierarchy are specialized or concrete; the classes at the higher levels are general or abstract.
  • Classes are mutually exclusive, which means that an item cannot belong to more than one class on each level of a classification scheme.

    Examples of classification schemes

    library classification schemes; biological taxonomies; directories on the WWW; linguistic thesauri; philosophical classification schemes


  • Categories are loose groupings of items; they are less rigid than classes.
  • They are non-exclusive, which means that an item can belong to many categories at the same time.
  • The boundaries among categories are not fixed, which means that it is sometimes not clear whether an item belongs to a category or not.
  • Categories usually have some prototypical members that are at the conceptual "center" of a category. The membership is graded, which means some items are "better" members of categories than others.
  • There may not be features that are shared by all members of a category, but members of a category usually show some similarity to each other.

    The cultural and cognitive nature of classification and categorization

    Classification and categorization usually depend on a social, linguistic and cultural context. It is therefore very difficult (or impossible) to build "universal" schemes that cover all human knowledge.

    Examples: Labov's cups; Berlin & Kay's basic color terms; prototypical class/category members, such as robin, pigeon for "bird"; chair, table for "furniture"

    Philosophical theories of classification and concepts

    Philosophical theories of classification: Aristotle; Smith & Medin.
    Rosch's "Prototype Theory" for categories.