Browsing by Author "Coetzer, Simone"

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    Repairing classical ontologies using defeasible reasoning techniques
    (Stellenbosch : Stellenbosch University, 2021-03) Coetzer, Simone; Britz, Katarina; Stellenbosch University. Faculty of Arts and Social Sciences. Dept. of Information Science.
    ENGLISH SUMMARY : Ontologies provide knowledge engineers with the ability to represent and encode knowledge in a formal language so that it can be reasoned over by a computer. Notable benets include the ability to source new knowledge by making statements that are implicitly deduced explicitly available to the end-user, to classify individuals or instances and to check the addition of new knowledge for logical consistency. Given the nature and goal of ontologies, a successful application of ontologies relies on (1) representing as much accurate and relevant domain knowledge as possible, (2) while maintaining logical consistency. As the successful implementation of a real-world ontology is likely to contain many concepts and intricate relationships between the concepts, it is necessary to follow a methodology for debugging and rening the ontology. A myriad of ontology debugging approaches (some of them instantiated in tools) have been developed to help the knowledge engineer pinpoint the cause of logical inconsistencies and rectify them in a strategic way. Rodler (2015) and Schekotihin et al. (2018) build out the ontology debugging basics by introducing an interactive ontology debugging methodology: this interactive ontology debugging framework, which has also been implemented as a Prot eg e plug-in, OntoDebug, methodically and iteratively asks users queries to narrow down the inconsistency to just one diagnosis, at which time the user can make a more informed decision about how to repair the diagnosis. This approach guides the user in the debugging process. We show however that this approach can sometimes lead to unintuitive results, which may then lead the knowledge engineer to opt for deleting potentially crucial and nuanced knowledge. This is due to the focus of the interactive ontology debugging approach to be on classical, monotonic knowledge bases { and indeed, in the classical/ monotonic sense, it is only by deletion, not extension of the knowledge base, that coherence can be obtained. However, it may at times be desirable to deal with the unintuitive results produced by weakening rather than deleting faulty axioms. We provide a methodological and design foundation for weakening faulty axioms in a strategic way using defeasible reasoning tools. Our methodology draws from Rodler's (2015) interactive ontology debugging approach which not only localises faulty axioms but provides the knowledge engineer with a strategic way of resolving them by presenting the root cause inconsistencies rst. We extend this approach by creating a methodology to systematically nd con ict resolution recommendations. Importantly, our goal is not to convert a classical ontology to a defeasible ontology { therefore we do not use defeasible reasoning support through, for example, the computation of rational closure. Rather, we use the denition of exceptionality of a concept, which is central to the semantics of defeasible description logics, and the associated algorithm (as can be found in Britz et al. 2019) to determine the extent of a concept's exceptionality (their ranking); then, starting with the statements containing the most general concepts (the least exceptional concepts) weakened versions of the original statements are constructed; this is done until all inconsistencies have been resolved.