Web Ontology Language (OWL) Abstract Syntax and Semantics

W3C Working Draft 8 November 2002

This version:: http://www.w3.org/TR/2002/WD-owl-semantics-20021108/
Latest version:: http://www.w3.org/TR/owl-semantics/
Previous version:: http://www.w3.org/TR/2002/WD-owl-absyn-20020729/
Editors:: Peter F. Patel-Schneider, Bell Labs Research
Patrick Hayes, IHMC, University of West Florida
Ian Horrocks, Department of Computer Science, University of Manchester
Frank van Harmelen, Department of Artificial Intelligence, Vrije Universiteit Amsterdam

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The OWL Web Ontology Language is being designed by the W3C Web Ontology Working Group as a revision of the DAML+OIL web ontology language. This description of OWL contains a high-level abstract syntax for both OWL and OWL Lite, a subset of OWL. A model-theoretic semantics is given to provide a formal meaning for OWL ontologies (or knowledge bases) written in the abstract syntax. A model-theoretic semantics in the form of an extension to the RDFS model theory is also given to provide a formal meaning for OWL ontologies written as n-triples. A mapping from the abstract syntax to n-triples is given and the two model theories are shown to have the same consequences on OWL ontologies that can be written in the abstract syntax.

Status of this document

This is a W3C Web Ontology Working Group Working Draft produced 8 November 2002 as part of the W3C Semantic Web Activity (Activity Statement). The Working Group maintains a list of issues, which are resolved by decisions of the group. This document incorporates decisions made thru approximately October 2002, and proposes design choices for a number of the remaining open issues.

This document is being released for review by W3C Members and other interested parties to encourage feedback and comments.

See also patent disclosures related to this work.

This is a public W3C Working Draft and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite as other than "work in progress". A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.

Comments on this document are invited and should be sent to the public mailing list public-webont-comments@w3.org. An archive of comments is available at http://lists.w3.org/Archives/Public/public-webont-comments/ . General discussion of related technology is welcome in www-rdf-logic.

1. Introduction

The W3C Web Ontology Working Group (WebOnt) is tasked with producing a web ontology language extending the reach of XML, RDF, and RDF Schema. This language, called OWL, is based on the DAML+OIL web ontology language [DAML+OIL].

This document contains several interrelated specifications of the several styles of OWL. First, Section 2 contains a high-level, abstract syntax for both OWL Lite, a subset of OWL [OWL Features], and a fuller style of using OWL, sometimes called OWL/DL. This document, however, defines neither a presentation syntax nor an exchange syntax for OWL. The official exchange syntax for OWL is RDF/XML; a document defining how RDF is used to encode OWL is the subject of the OWL Reference document [OWL Reference]. A mapping from the abstract syntax to n-triples [RDF Tests] is, however, provided in Section 4.

This document contains two formal semantics for OWL. One of these semantics, defined in Section 3, is a direct, standard model-theoretic semantics for OWL ontologies written in the abstract syntax. The other, defined in Section 5, is an extension of the RDFS model-theoretic semantics that starts with n-triples and provides a semantics for OWL ontologies written in the official OWL exchange syntax. Two versions of this second semantics are provided, one that corresponds more-closely to the direct semantics and one that can be used in cases where classes need to be treated as individuals. These two versions are actually very close, only differing in how they divide up the domain of discourse.

Appendix A contains a proof that the two different semantics have the same consequences on OWL ontologies that correspond to abstract OWL ontologies that separate OWL individuals, OWL classes, OWL properties, and the RDF, RDFS, and OWL structural vocabulary. For such OWL ontologies the direct model theory is authoritative and the RDFS-compatible model theory is secondary.

Finally a few examples of the various concepts defined in the document are presented in Appendix B.

1.1. Differences from DAML+OIL

The language described in this document is very close to DAML+OIL. The exchange syntax for OWL, as defined in the OWL Reference document [OWL Reference], has only minor changes from DAML+OIL. Even the abstract syntax can be viewed as an abstract syntax for DAML+OIL.

There are also a number of minor differences between OWL and DAML+OIL including a number of changes to the names of the various constructs, as mentioned in Appendix A of the OWL Reference Description [OWL Reference]. These naming changes may indicate potential changes to the preferred names in the concrete syntax for OWL, but the intent of WebOnt is to maintain the DAML+OIL names to the maximum extent reasonable.

1.2. Stances Taken on Open OWL Issues

This document takes stances on several open working group issues, some of which are detailed below. All working group issues are discussed in the issue listing document [OWL Issues].

Acknowledgments

The Joint US/EU ad hoc Agent Markup Language Committee developed DAML+OIL, which is the direct precursor to OWL. Many of the ideas in DAM+OIL and thus in OWL are also present in the Ontology Inference Layer (OIL). Many of the other members of the W3C Web Ontology Working Group have had substantial input into this document.