Defeasible modes of inference: A preferential perspective

Abstract

Historically, approaches to defeasible reasoning have been concerned mostly with one aspect of defeasibility, viz. that of arguments, in which the focus is on normality of the premise. In this paper we are interested in another aspect of defeasibility, namely that of defeasible modes of reasoning. We do this by adopting a preferential modal semantics that we defined in previous work and which allows us to refer to the relative normality of accessible worlds. This leads us to define preferential versions of the traditional notions of knowledge, beliefs, obligations and actions, to name a few, as studied in modal logics. The resulting preferential modal logics make it possible to capture, and reason with, aspects of defeasibility heretofore beyond the reach of modal formalisms.