What Is OWL (and Why Should I Care)?

OWL is the new ontology language produced by the W3C Web Ontology Working Group. OWL is thus poised to be a major formalism for the design and dissemination of ontology information, particularly in the Semantic Web. OWL has influences from several communities, including the RDF community, the Description Logic community, and the frame community. These influences resulted in a wide variety of requirements on OWL, some of which appear to be conflicting. OWL contains innovative solutions to several of these apparent conflicts, but it has not been possible to completely satisfy all the desired requirements for OWL. The talk will describe the development and design of OWL, concentrating on what makes OWL important, the relationship of OWL to other formalisms, the place of OWL in the Semantic Web, the innovative solutions that were required in its design, and the impact of the conflicting requirements on OWL. I will propose a different foundation for the Semantic Web, one that I think would allow for easier and better development of new formalisms for the Semantic Web. OWL is poised to be a major part of the Semantic Web, but what is OWL and how does it fit into the Semantic Web? Answers to these questions are intimately intertwined with the history and development of OWL, in particular the constraints that were placed on the design of OWL because of its positioning within the W3C’s vision of the Semantic Web. OWL (Dean et al. 2004) is the W3C recommendation that provides ontology services for the Semantic Web. Because OWL is part of W3C’s Semantic Web, the official exchange syntax for OWL is XML/RDF (Beckett 2004), a way of writing RDF (Manola & Miller 2004) in XML. Because OWL is an ontology language descended from Description Logics, OWL has a model-theoretic semantics (Patel-Schneider, Hayes, & Horrocks 2004) that provides the official meaning for OWL documents. Again because OWL is part of W3C’s Semantic Web, the model-theoretic semantics for OWL is compatible with the model-theoretic semantics provided for RDF (Hayes 2004). An ontology language (in this context, at least) is a language in which it is possible to provide information about the different kinds of objects in the domain of discourse (i.e., the part of the world that is of interest). Collections of such Copyright 2004, American Association for Artificial Intelligence (www.aaai.org). All rights reserved. information are called ontologies. An ontology thus provides a way of talking about the world. There are different kinds of ontologies that have been proposed. These range from ontologies that structure the fundamental kinds of objects (for example, dividing the world into physical objects, imaginary objects, and so on) through ontologies that provide the basic for large areas of knowledge (for example, providing an ontology for electronic commerce) to ontologies for particular application domains (for example, dividing travel services into airline flight reservation services, car rental services, and so on). There have also been many different kinds of language proposed as ontology languages. These languages have ranged from very powerful languages in which just about anything can be said, such as higher-order logics, through less expressive languages in which only certain kinds of things can be said, such as Description Logics, down to very simple languages, such as simple generalization taxonomies. OWL can be used to build most kinds of ontologies, but it is not as expressive as higher-order or even first-order logic, and thus certain kinds of ontologies cannot be built in OWL. In particular, OWL is ill-suited to create and reason with an ontology for OWL itself. OWL has been influenced from three sides. Because OWL is part of W3C’s Semantic Web, OWL has been heavily influenced by W3C’s vision of the Semantic Web, as layers built on top of RDF. From this vision comes the official OWL exchange syntax, namely RDF/XML. More importantly, OWL has a very close connection to the semantics of RDF with one version of OWL being a semantic extension of RDF Schema (Brickley & Guha 2004), itself a semantic extension of RDF. Because OWL is closely related to Description Logics, OWL has many features that come from this family of knowledge representation systems. Description Logics provide the main knowledge-structuring capabilities of OWL. Further, the semantics of the knowledge structuring capabilities of OWL come directly from Description Logics, so that a construct in OWL has the same model-theoretic meaning as its analogue has in other Description Logics. OWL has also been influenced by some of the knowledgestructuring capabilities of frame systems. From this influence comes some of the difference in syntax between OWL and most other Description Logics. In particular, OWL con-