DEFAULT REASONING AND GENERICS

There are numerous logical formalisms capable of drawing conclusions u sing default rules. Such systems, however, do not normally determine w here the default rules come from; i.e., what it is that makes Birds fl y a good rule, but Birds drive trucks a bad one. Generic sentences suc h as Birds fly are often used informally to describe default rules. I propose to take this characterization seriously, and claim that a defa ult rule is adequate iff the corresponding generic sentence is true. T hus, if we know that Tweety is a bird, we may conclude by default that Tweety flies, just in case Bird fly is a true sentence. In this paper , a quantificational account of the semantics of generic sentences is presented. It is argued that a generic sentence is evaluated not in is olation, but with respect to a set of relevant alternatives. For examp le, Mammals bear live young is true because among mammals that bear li ve young, lay eggs, undergo mitosis, or engage in some alternative for m of procreation, the majority bear live young. Since male mammals do not procreate in any form, they do not count. Some properties of alter natives are presented, and their interactions with the phenomena of fo cus and presupposition is investigated. It is shown how this account o f generics can be used to characterize adequate default reasoning syst ems, and several desirable properties of such systems are proved. The problems of the automatic acquisition of rules from natural language a re discussed. Because rules are often explicitly expressed as generics , it is argued that the interpretation of generic sentences plays a cr ucial role in this endeavor, and it is shown how the theory presented here can facilitate such interpretation.