Well-Founded Optimism: Inheritance in Frame-Based Knowledge Bases

F-logic is a popular formalism for knowledge-intensive applications and, especially, for ontology management in Semantic Web. However, the original F-logic’s semantics for inheritance suffers from a number of anomalies when inheritance and deduction closely interact. This work rectifies this problem and develops a natural model-theoretic semantics for inheritance in frame-based knowledge bases, which supports inference by inheritance as well as inference via rules. Inference by inheritance supports a multitude of features, such as overriding and nonmonotonic multiple inheritance, meta programming, and dynamic inheritance hierarchies — the features that are fundamental to advanced knowledge management. This semantics has been effectively implemented in the Flora-2 system which is extensively used in a number of projects. To the best of our knowledge, this work is the only model-theoretic semantics for nonmonotonic multiple inheritance that applies to general, unrestricted frame-based knowledge bases and has several independent characterizations, which testifies to its naturalness and robustness. The problems discussed in this paper are inherent in any logic-based system that supports inheritance and deductive rules and our techniques apply to such systems. In particular, they apply to DAML+OIL extended with rules and inheritance.