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

OWL Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...

. According to the

Type–token distinction The type–token distinction is the difference between naming a ''class'' (type) of objects and naming the individual ''instances'' (tokens) of that class. Since each type may be exemplified by multiple tokens, there are generally more tokens than ...

, 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 Instances.

Relationships

Instantiation

The instantiation 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

subsume Subsumption may refer to: * A minor premise in symbolic logic (see syllogism) * The Liskov substitution principle in object-oriented programming * Subtyping in programming language theory * Subsumption architecture in robotics * A subsumption r ...

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'.

Relationships

Instantiation

Subsumption

References

See also