Ontolingua: a mechanism to support portable ontologies

An ontology is a set of deenitions of content-speciic knowledge representation prim-itives: classes, relations, functions, and object constants. Ontolingua is mechanism for writing ontologies in a canonical format, such that they can be easily translated into a variety of representation and reasoning systems. This allows one to maintain the ontol-ogy in a single, machine-readable form while using it in systems with diierent syntax and reasoning capabilities. The syntax and semantics are based on the KIF knowledge interchange format 11]. Ontolingua extends KIF with standard primitives for deening classes and relations, and organizing knowledge in object-centered hierarchies with inheritance. The Ontolingua software provides an architecture for translating from KIF-level sentences into forms that can be eeciently stored and reasoned about by target representation systems. Currently, there are translators into LOOM, Epikit, and Algernon, as well as a canonical form of KIF. This paper describes the basic approach of Ontolingua to the ontology sharing problem, introduces the syntax, and describes the semantics of a few ontological commitments made in the software. Those commitments, which are reeected in the on-tolingua syntax and the primitive vocabulary of the frame ontology, include: a distinction between deenitional and nondeenitional assertions; the organization of knowledge with classes, instances, sets, and second-order relations; and assertions whose meaning depends on the contents of the knowledge base. Limitations of Ontolingua's \conser-vative" approach to sharing ontologies and alternative approaches to the problem are discussed.