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In the Semantic Web, a metaclass is a class whose instances are classes. Metaclasses are supported in the ontology language OWL and the data-modeling vocabulary RDFS. Similar to their role in programming languages, metaclasses in Semantic Web languages can have properties otherwise applicable only to individuals, while retaining the same class's ability to be classified in a concept hierarchy. This enables knowledge about instances of those metaclasses to be inferred by semantic reasoners using statements made in the metaclass. Metaclasses thus enhance the expressivity of knowledge representations in a way that can be intuitive for users. While classes are suitable to represent a population of individuals, metaclasses can, as one of their feature, be used to represent the conceptual dimension of an ontology. Metaclasses are often modeled by setting them as the object of claims involving rdf:type and rdfs:subClassOf—built-in properties commonly referred to as ''instance of'' and ''subclass of''. ''Instance of'' entails that the subject of the claim is an instance, i.e. an individual that is a member of a class. ''Subclass of'' entails that the subject is a class. In the context of ''instance of'' and ''subclass of'', ordinary can be are the object of ''instance of'' claims used on a class, but ordinary classes are not. (where in a claim Bob ''instance of'' Human, Bob is the subject, and Human the object).OWL 2 DL supports metaclasses by a feature called punning,〔(【引用サイトリンク】 title=OWL 2 Web Ontology Language New Features and Rationale (Second Edition) )〕 in which one entity is interpreted as two different types of thing—a class and an individual—depending on its syntactic context. For example, through punning, an ontology could have a concept hierarchy such as Harry the eagle ''instance of'' golden eagle, golden eagle ''subclass of'' bird, and golden eagle ''instance of'' species. In this case, the punned entity would be golden eagle, because it is represented as a class (second claim) and an instance (third claim); whereas the metaclass would be species, as it has an instance that is a class. Punning also enables other properties that would otherwise be applicable only to ordinary instances to be used directly on classes, for example "golden eagle ''conservation status'' least concern."Having arisen from the fields of knowledge representation, description logic and formal ontology, Semantic Web languages have a closer relationship to philosophical ontology than do conventional programming languages such as Java or Python. Accordingly, the nature of metaclasses is informed by philosophical notions such as abstract objects, the abstract and concrete, and type-token distinction. Metaclasses permit concepts to be construed as tokens of other concepts while retaining their ontological status as types. This enables types to be enumerated over, while preserving the ability to inherit from types. For example, metaclasses could allow a machine reasoner to infer from a human-friendly ontology how many elements are in the periodic table, or, given that number of protons is a property of chemical element and isotopes are a subclass of elements, how many protons exist in the isotope hydrogen-2. == Examples == Following the type-token distinction, real world objects such as Abraham Lincoln or the planet Mars are regrouped into classes of similar objects. Abraham Lincoln is said to be an ''instance of'' human, and Mars is an ''instance of'' planet. This is a kind of is-a relationship. Metaclasses are class of classes, such as for example the nuclide concept. In chemistry, atoms are often classified as elements and, more specifically, isotopes. The glass of water one last drank has many hydrogen atoms, is of which is an ''instance of'' hydrogen. Hydrogen itself, a class of atoms, is an ''instance of'' nuclide. Nuclide is a class of classes, hence a metaclass. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Metaclass (Semantic Web)」の詳細全文を読む スポンサード リンク
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