Ontologies and controlled vocabularies


The primary meaning of ontology (from the Greek οντος, “pertaining to being or what exists” and λóγος “science, study, theory”) is a branch of metaphysics occupied with the study of what is. In his writing, Aristotle named metaphysics as what was, literally speaking, after physics.  This realm of knowledge corresponded to the study of what he called first philosophy, or ontology, meaning the study of what is, is real, or exists.

Ontology is not only concerned with entities or the things that exist, but also with the way in which the entities that exist relate to one another. This idea of a science of entities and their relationships started to be used in the formal language of computational and information sciences to designate a set of definitions for classes, types, attributes, properties and relationships among entities that act within a specified domain of reality and knowledge. Indeed, this is a practical application of the concepts of philosophical ontology.

What the philosophical and the IT perspectives have in common is the representation of entities, ideas and events, together with their properties and relationships, according to a categorisation system. IT professionals, however, concentrate their efforts on “closing” ontologies by representing them with controlled vocabularies that can be utilised by computer systems.

In this sense, an ontology is a set of individuals (instances or objects); classes (sets, collections, concepts, types of objects or kinds of things); attributes (aspects, properties, features, characteristics or parameters that objects and classes can have); relations (way in which classes and individuals may be related to each other); functions (complex structures formed from certain relations that can be used in place of an individual term in a statement); restrictions (these establish formal descriptions of what must be true in order for some assertion to be accepted as input); rules (statements in the form of an if-then (antecedent-consequent) sentence that describe the logical inferences that can be drawn from an assertion in a particular form); axioms (assertions, including rules, in a logical form that together comprise the overall theory that the ontology describes in its domain of application) and events (the changing of attributes or relations).

Ontologies are usually encoded using standard ontology languages such as Ontology Web Language (OWL), that allow classes and their sets of attributes to be described. For example, the class “person” and its set of attributes: having a name and surname, a place of birth, a date of birth, etcetera. The Resource Data Format (RDF) is a file that identifies a specific individual from a class such as, “Diego de Silva y Velázquez”, born in Seville, on such and such a date, who was a painter, and so on.

Domain ontologies

A domain ontology (or domain-specific ontology) represents concepts belonging to a specific part of our world reality and may hence be considered to involve highly specialised knowledge. Similarly to the way in which ontological objectives of information sciences and technology aspire to close and control vocabularies as much as possible, domain-specific ontologies are the natural consequence of efforts to represent and compute digital resource content. In this sense, the individual meaning of a term within a domain is given by the ontology’s domain.

Because concept-based domain ontologies represent their concepts in a very specific way, the ontologies do not tend to overlap, or rather, they may be mutually exclusive.

Ontological hybridization

On the other hand, reality as a whole has a notable tendency toward continuity, and the domains in which the world is organised tend to be more mixed than our controlled vocabularies. This is why world systems, like any organisation or human institution, require expanded domain ontologies or hybrid ontologies; these are the result of the blending and integration of different domain ontologies in a more general representation. Hybrid ontologies require the design of an upper ontological foundation on top of the foundation of controlled vocabularies that are built from different ideas originating from all over the world, sometimes in different languages. These always originate from distinct or external cultural sites.

Mixing ontologies is an artistic process that attempts to digitalise a domain or sector of reality that exceeds its controlled vocabularies’ capacity for representation.