OWL plus Rules = ...?
Description Logics (DL)-the logics underpinning the Web Ontology Language OWL-and rules-the formalism underlying the Semantic Web Rule Language SWRL and the Rule Interchange Format RIF-are currently the most prominent paradigms used for modeling knowledge for the Semantic Web. While both of these approaches are based on classical logic, their paradigms differ significantly, so that naive combinations result in undesirable properties such as undecidability. Recent work has shown that many rules can in fact be expressed within the DL paradigm, and therefore within the OWL language.
In this tutorial, we formally identify the set of rules expressible in OWL and show how to translate these into equivalent sets of OWL axioms. We also discuss different extensions of OWL that allow us to express arbitrary Datalog rules and even non-monotonic rules in DL syntax. The latter enable us to include some non-classical reasoning features, such as defaults and exceptions.