AN EPISTEMIC OPERATOR FOR DESCRIPTION LOGICS

Description logics (also called terminological logics, or concept lang uages) are fragments of first-order logic that provide a formal accoun t of the basic features of frame-based systems. However, there are asp ects of frame-based systems-such as nonmonotonic reasoning and procedu ral rules-that cannot be characterized in a standard first-order frame work. Such features are needed for real applications, and a clear unde rstanding of the logic underlying them is necessary for principled imp lementations. We show how description logics enriched with an epistemi c operator can formalize such aspects. The logic obtained is a fragmen t of a. first-order nonmonotonic modal logic. We show that the epistem ic operator formalizes procedural rules, as provided in many knowledge representation systems, and enables sophisticated query formulation, including various forms of closed-world reasoning. We provide an effec tive procedure for answering epistemic queries posed to a knowledge ba se expressed in a description logic and extend this procedure in order to deal with rules. We also address the computational complexity of r easoning with the epistemic operator, identifying cases in which an ap propriate use of the epistemic operator can help in decreasing the com plexity of reasoning. (C) 1998 Elsevier Science B.V.