Nikos Dimaresis
A System for Modal and Deontic Defeasible Reasoning

Defeasible logic is a simple and efficient rule-based nonmonotonic reasoning ap-proach, that has been shown useful for various applications areas. Recently defeasible logic has been used in applications to the Semantic Web. The Semantic Web is an extension of the current Web, in which information is given well-defined meaning, and its development proceeds in layers, each layer being on top of other layers. Now that the layers of metadata (RDF) and ontology (OWL) have reached sufficient maturity, the next step will be the logic and proof layer and an important focus is on rule languages for the Semantic Web. While initially the focus has been on monotonic rule systems, nonmonotonic rule systems are increasingly gaining attention. The first source of motivation for our work is the modelling of multi-agent systems based on cognitive and social models, where an agent behavior is determined as an interplay between mental attitudes and normative aspects. Commonly, these aspects are logically captured through the use of modal logics, which are by definition monotonic. Reasoning about intentions and other mental attitudes has defeasible nature, and defeasibility is a key aspect for normative reasoning. The second important source for motivation for our work is the modelling of policies. Defeasible logic is the nonmonotic reasoning approach that would be suitable solution for the requirements that arise from the specific nature of policies and especially of business rules. In our work we use and develop an extend variant of defeasible logic, that uses modal and deontic operators. This is a suitable formalism that can deal with the motivational components of our work and can capture their nonmonotonic behavior. For the purposes of modelling policies sufficiently, we will introduce an additional deontic operator for expressing “permission”. We implement a nonmonotonic rule based system, based on this formalism, which integrates with the Semantic Web, as it reasons with the standards of RDF and RDF Schema. The core of the system consists of a logic metaprogram that implements the extension of defeasible logic.

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