Deontic Logic Viewed as Defeasible Reasoning

Abstract

The notion of defeasibility in normative reasoning has been discussed by legal philosophers such as Gardner (1987), Hart (1948, 1958), and MacCormick (1974). For instance, MacCormick, in his 1974 article and also in an unpublished research note “Defeasibility in Law and Logic,” stated that an “institutional fact,” supporting the validity of a certain legal consequence, is the “ordinarily necessary and presumptively sufficient conditions” of the consequence and thus may be “subject to some kind of invalidating intervention” that brings about its defeasance. Similarly, Gardner (1987) observed “defeasibility of legal consequences.” Alchourron and Makinson (1981) proposed hierarchical reasoning about norms, which showed defeasibility aspects of norms. Ross (1930) distinguished “prima facie norms” and “actual norms,” in which a prima facie norm becomes an actual norm after “all things are considered” and there is no reason to rebut it (Bonevac, 1987).