Ontology

Ontology

Since the direct representation of existence and the universal representation of thought are both based on ontologies, a brief review of ontology is in order.

According to Tom Gruber, an ontology is an explicit specification of a conceptualization (Gruber, 1993). That is, an ontology is a description (like a formal specification of a program) of the concepts and concept relationships that are of interest in a domain of discourse. The term “ontology” is borrowed from philosophy, where an ontology is a systematic account of existence.
For knowledge-based systems, what “exists” is exactly that which can be represented.

Formally, an ontology is the statement of a logical theory; i.e, an ontology is the (representation / description / encoding) of a logical theory. Most ontologies defined to date have been based on indirect representations. Thought is represented in terms of and relative to each agents’ universal ontolology.

When we represent the knowledge of a domain in a declarative formalism, we call the set of objects that can be represented the universe of discourse. This set of objects, and the describable relationships among them, are reflected in the representational vocabulary with which a knowledge-based program represents knowledge. Thus, we can describe the ontology of a program by defining a set of representational terms. In such an ontology, definitions associate the names of entities in the universe of discourse (e.g., classes, relations, functions, or other objects) with human-readable text describing what the names are meant to denote, and formal axioms that constrain the interpretation and well-formed use of these terms.

We use common (shared) ontologies to describe ontological commitments for a set of agents so that they can communicate about a domain of discourse without necessarily operating on a globally shared theory. We say that an agent commits to an ontology if its observable actions are consistent with the definitions in the ontology. Pragmatically, a common ontology defines the vocabulary with which queries and assertions are exchanged among agents. Ontological commitments are agreements to use the shared vocabulary in a coherent and consistent manner. The agents sharing a vocabulary need not share a knowledge base; each knows things the other does not, and an agent that commits to an ontology is not required to answer all queries that can be formulated in the shared vocabulary.

In short, a commitment to a common ontology is a guarantee of consistency, but not completeness, with respect to queries and assertions using the vocabulary defined in the ontology.

A domain ontology (or domain-specific ontology) models a specific domain, or part of the world. It represents the particular meanings of terms as they apply to that domain. For example, the word ‘card’ has many different meanings. An ontology about the domain of poker would model the ‘playing card’ meaning of the word, while an ontology about the domain of computer hardware would model the ‘punch card’ and ‘video card’ meanings.

In information science, an upper ontology (top-level ontology, or foundation ontology) is an attempt to create an ontology which describes very general concepts that are the same across all domains. The aim is to have a large number of ontologies accessible under this upper ontology. It is usually a taxonomy of entities, relationships, and axioms that attempts to describe the representation of those general entities that do not belong to a specific problem domain.