A category, so called because it is possible to provide necessary and jointly sufficient conditions for determining that an entity belongs to a particular category. Examples of such categories include the categories bachelor and odd number.

Nevertheless, since the advent of Eleanor Rosch's work on prototype theory it has become clear that even classical categories exhibit typicality effects. For instance, some members of the odd number category such as 1, 3, 5 and 9 are typically judged as being better examples of the category than high odd numbers such as 1001.