Yasuhiro Katagiri, ATR Media Integration & Communications Research Laboratories, Japan
Use of confirmations in dialogue is mundane and ubiquitous, but theoretically it has been a puzzle how confirmations serve their supposed end, namely to secure the establishment of mutual understandings between dialogue participants. We propose a solution to this confirmation puzzle by introducing the notion of defeasible mutual knowledge. The notion is based upon our intuition that mutual beliefs obtained in message exchanges are inherently of nonmonotonic nature in the sense that they are revocable by the negating evidences obtained later. We point out that there are two aspects to the notion of defeasible knowledge: optimism and error discovery. We give an analysis of defeasible knowledge within the framework of an epistemic temporal modal logic over distributed systems, and propose a set of conditions for defeasible mutual knowledge. We examine a simple message transmission example and show that confirmations are necessary to obtain defeasible mutual knowledge. We also examine organizational structures for defeasible mutual knowledge for larger groups.