What Are Ontologies , and Why Do We Need Them ?

THEORIES IN AI FALL INTO TWO broad categories: mechanism theories and content theories. Ontologies are content theories about the sorts of objects, properties of objects, and relations between objects that are possible in a specified domain of knowledge. They provide potential terms for describing our knowledge about the domain. In this article, we survey the recent development of the field of ontologies in AI. We point to the somewhat different roles ontologies play in information systems, naturallanguage understanding, and knowledgebased systems. Most research on ontologies focuses on what one might characterize as domain factual knowledge, because knowlede of that type is particularly useful in natural-language understanding. There is another class of ontologies that are important in KBS—one that helps in sharing knoweldge about reasoning strategies or problemsolving methods. In a follow-up article, we will focus on method ontologies.