Abstract
The word "ontology" presently encompasses two different meanings. In the philosophical sense, ontology has been defined since the days of Aristotle as the "science of being," also called "metaphysics." In this sense, ontology is concerned with categorizing discrete entities in reality and the relationships that hold between those entities (Aristotle 1952). Ontology, in the tradition of philosophical realism, aims to describe every day, mesoscopic entities and relations as they are in themselves, and not from the specific viewpoint of some particular domain or conceptual schema. The type of entities realist ontology concerns are those that exist "out there" in the world (such as tables, chairs, mountains, streams ..