Introduction: Progress in formal commonsense reasoning

This special issue consists largely of expanded and revised versions of selected papers of the Fifth International Symposium on Logical Formalizations of Commonsense Reasoning (Common Sense 2001), held at New York University in May 2001.1,2 The Common Sense Symposia, first organized in 1991 by John McCarthy and held roughly biannually since, are dedicated to exploring the development of formal commonsense theories using mathematical logic. Commonsense reasoning is a central part of human behavior; no real intelligence is possible without it. Thus, the development of systems that exhibit commonsense behavior is a central goal of Artificial Intelligence. It has proven to be more difficult to create systems that are capable of commonsense reasoning than systems that can solve “hard” reasoning problems. There are chess-playing programs that beat champions [5] and expert systems that assist in clinical diagnosis [32], but no programs that reason about how far one must bend over to put on one’s socks. Part of the difficulty is the all-encompassing aspect of commonsense reasoning: any problem one looks at touches on many different types of knowledge. Moreover, in contrast to expert knowledge which is usually explicit, most commonsense knowledge is implicit. One of the prerequisites to developing commonsense reasoning systems is making this knowledge explicit. John McCarthy [25] first noted this need and suggested using formal logic to encode commonsense knowledge and reasoning. In the ensuing decades, there has been much research on the representation of knowledge in formal logic and on inference algorithms to