# Class (knowledge representation)

In knowledge representation, a class is a collection of individuals or objects.[1] A class can be defined either by extension, or by intension, using what is called in some ontology languages like OWL. If we follow 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 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's a bird, he's a meat eater and so on. It's a special kind of is a relationship. It's noted Concept assertion (${\displaystyle :}$) in Description logics, a family of logic based on classes, class assertion [2]

### Subsumption

Classes can 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's called subclassof.[2]

## References

1. ^ Diego Calvanese; Giuseppe De Giacomo; Maurizio Lenzerini (2002). Description Logics: Foundations for Class-based Knowledge Representation. Logic in Computer Science.
2. ^ a b