In
knowledge representation
Knowledge representation and reasoning (KRR, KR&R, KR²) is the field of artificial intelligence (AI) dedicated to representing information about the world in a form that a computer system can use to solve complex tasks such as diagnosing a medic ...
, a class is a collection of individuals or objects.
A class can be defined either by
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* Ext ...
(specifying members), or by
intension
In any of several fields of study that treat the use of signs — for example, in linguistics, logic, mathematics, semantics, semiotics, and philosophy of language — an intension is any property or quality connoted by a word, phrase, or ano ...
(specifying conditions), using what is called in some ontology languages like Web Ontology Language, OWL. According to the Type–token distinction, the ontology is divided into individuals, who are real worlds objects, or events, and types, or classes, who are sets of real world objects. Class expressions or definitions gives the properties that the individuals must fulfill to be members of the class. Individuals that fulfill the property are called Instance (computer science), Instances.
Relationships
Instantiation
The instantiation relation (mathematics), relationship is a relation between objects and classes. We say that an object O, say ''Harry the eagle'' is an instance of a class, say ''Eagle''. ''Harry the eagle'' has all the properties that we can attribute to an eagle, for example his parents were eagles, he is a bird, he is a meat eater and so on. It is a special kind of is a relationship. Its noted Concept assertion (
) in Description logics, a family of logic based on classes, class assertion
Subsumption
Classes can is a, subsume each other. We say usually that if
A and
B are classes, and all
A instances are also
B instances, then B subsumes A, or A is a subclass of B, for example in the OWL Language it is called 'subclassof'.
References
See also
* Metaclass (Semantic Web)
* Ontology (information science), Ontology
* Ontology components
* Description logic
* Type-token distinction
Knowledge representation
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