Epistemic Querying of OWL Knowledge Bases Anees ul Mehdi, Sebastian Rudolph, and Stephan Grimm Institute AIFB, Karlsruhe Institute of Technology, DE {sebastian.rudolph,anees.mehdi}@kit.edu Forschungszentrum Informatik Karlsruhe, DE grimm@fzi.de Abstract. Epistemic querying extends standard ontology inferencing by allowing for deductive introspection. We propose a technique for epis- temic querying of OWL 2 ontologies not featuring nominals and univer- sal roles by a reduction to a series of standard OWL 2 reasoning steps thereby enabling the deployment of off-the-shelf OWL 2 reasoning tools for this task. We prove formal correctness of our method, justify the omission of nominals and universal role, and provide an implementation as well as evaluation results. 1 Introduction Ontologies play a crucial role in the Semantic Web and the Web Ontology Language (OWL, [9]) is the currently single most important formalism for web-based semantic applications. OWL 2 DL – the most comprehensive version of OWL that still allows for automated reasoning – is based on the description logic (DL) SROIQ [6]. Querying ontologies by means of checking entailment of axioms or instance retrieval is a crucial and promi- nent reasoning task in semantic applications. Despite being an expressive formalism, these standard querying capabilities with OWL ontologies lack the ability for introspection (i.e., asking what the knowledge base “knows” within the query language). Autoepistemic DLs cope with this problem and have been investigated in the context of OWL and Semantic Web. In particular, they allow for introspection of the knowledge base in the query language by means of epistemic operators, such as the K-operator (paraphrased as ”known to be”) that can be applied to concepts and roles. The K-operator allows for epistemic querying. E.g., in order to formu- late queries like ”known white wine that is not known to be produced in a French region” we could do an instance retrieval w.r.t. the DL concept KWhiteWine ⊓ ¬∃KlocatedIn .{FrenchRegion }.