<特集：モデル> 条件法論理に基づく情報更新の論理

Abstract

The purpose of this paper is to argue that hybrid formalism fits naturally into David Lewis's counterfactual logic and their introduction is a desirable step. This hybridization enables us to regard the inference "The pig is Mary; Mary is pregnant; therefore the pig is pregnant" as a process of updating local information (which depends on the given situation) by using global information (independent of the situation). Our hybridization also has the following technical merits: (i) It preserves the completeness and decidability of Lewis's logic; and (ii) it allows us to characterize the Limit Assumption by a proof-rule with some side-condition.