Ontologies provide knowledge engineers with the ability to represent and
encode knowledge in a formal language so that it can be processed or ‘reasoned
over’ by a computer [
Given the nature and goal of ontologies, a successful application of
ontologies relies on (1) representing as much relevant domain knowledge accurately
(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 the
ontology [
Although most ontology debugging approaches localise the faulty axioms,
they do not (to date) provide recommendations on how logical inconsistencies
can be resolved by weakening (instead of deleting) faulty axioms. We propose
a theoretical methodology for weakening faulty axioms in a strategic way using
defeasible reasoning tools. Our methodology draws from Rodler’s [
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 definition of
exceptionality of a concept, which is central to the semantics of defeasible DLs,
and the associated algorithm (as can be found in [
Coetzer and Britz 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. Consider how this methodology can be applied to the following set of statements in an example ontology:
When running the ranking algorithm, the concept ‘User’ is the most general exceptional concept; the concept ‘Staff’ is more exceptional than the concept of ‘User’. Starting with the least exceptional concept, ‘User’, we then weaken statements containing this concept on the left hand side of the axiom by using a conjunction between the current concept under investigation and the negation of the concept associated with it on the next level of exceptionality (Staff) - in this phase, the ontology is transformed as follows:
The ranking of all concepts is again calculated and the first exceptional concept, ‘User’ has now been resolved. Then, we move onto the next concept, Staff, which is now the most general exceptional concept. The same kind of transformation is performed on axioms containing ‘Staff’ on the left hand side (with the exclusion of axioms containing one of the previous exceptional statements on the right hand side):
The forward-looking goal of this research is to provide a methodological foundation which could, in future, lead to the development and implementation of an inconsistency resolution recommender tool that is fully integrated with the ontology development environment.
Debugging classical ontologies using defeasible reasoning tools