Body of Knowledge

Sometimes, ontology is defined as a body of knowledge describing some domain, typically a common sense knowledge domain, using a representation vocabulary as described above. In this case, an ontology is not only the vocabulary, but the whole ``upper'' knowledge base (including the vocabulary that is used to describe this knowledge base).

The typical example is the project CYC that defines its knowledge base as an ontology for any other knowledge based system. CYC is the name of a very large, multi-contextual knowledge base and inference engine. CYC is an early attempt to do symbolic AI on a massive scale by capturing common knowledge that is required to do tasks that are trivial for people, but very hard for computers. All of the knowledge in CYC is represented declaratively in the form of logical assertions. CYC contains over 400,000 significant assertions, which include simple statements of facts, rules about what conclusions to draw if certain statements of facts are satisfied, and rules about how to reason with certain types of facts and rules. New conclusions are derived by the inference engine using deductive reasoning. The CYC common sense knowledge can be used as a foundation of a knowledge base for any knowledge intensive system. In this sense, this body of knowledge can be viewed as an ontology of the knowledge base of the system.

Generally speaking, it is not possible to objectively distinguish what should be in ontology and what should be in knowledge base. It depends on the requirements of intended usage of the ontology. That is why the CYC view of ontology is not incompatible with the Gruber's definition, even when it may contain facts that one would classify as representing state of affairs.