class

In knowledge representation, a class is a collection of individuals or individuals objects. A class can be defined either by extension (specifying members), or by intension (specifying conditions), using what is called in some ontology languages like 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 Instances. Examples Some examples of classes: Person, the class of all people, or the abstract object that can be described by the criteria for being a person. Vehicle, the class of all vehicles, or the abstract object that can be described by the criteria for being a vehicle. Car, the class of all cars, or the abstract object that can be described by the criteria for being a car. Class, representing the class of all classes, or the abstract object that can be described by the criteria for being a class. Thing, representing the class of all things, or the abstract object that can be described by the criteria for being a thing (and not nothing). Definition Classes – concepts that are also called type, sort, category, and kind – can be defined as an extension or an intension. According to an extensional definition, they are abstract groups, sets, or collections of objects. According to an intensional definition, they are abstract objects that are defined by values of aspects that are constraints for being member of the class. The first definition of class results in ontologies in which a class is a subclass of collection. The second definition of class results in ontologies in which collections and classes are more fundamentally different. Classes may classify individuals, other classes, or a combination of both. Extensional or intensional definitions The classes of an ontology may be extensional or intensional in nature. A class is extensional if and only if it is characterized solely by its membership. More precisely, a class C is extensional if and only if for any class C', if C' has exactly the same members as C, then C and C' are identical. If a class does not satisfy this condition, then it is intensional.