An approach to default reasoning based on a first-order conditional logic: Revised report (Q1103607)

A logic N for default reasoning, based on an extension of classical first order logic with an operator ``if... then normally...'' and with possible worlds semantics is presented. N is more appropriate for representing common-sense intuition and information about defauls than other approaches. Two formulations of default inference relying on explicitly made assumptions of normality and relevance are shown to be equivalent with respect to their set of default inferences.