Thomas Agotnes (U. Bergen)
ABSTRACT. A lie is a statement that is false, or at least believed to be false, when it is announced. But the world after the lie is not the same as the world before the lie, so is the statement necessarily still false after the lie is announced - is the lie still a lie? This talk is about true lies. These are "self-fulfilling" lies that become true when they are made. The analysis is based on formal modal epistemic logic. True lies are conceptually related to Moore sentences, sentences that are true but become false when they are announced, but the exact relationship between the two types of sentences is not trivial. I will also discuss impossible lies (lies that stay false when announced) as well as the relationships to successful formulas (truths that stay truths when announced) and self-refuting truths (truths that become false when announced). The talk is based on joint work with Hans van Ditmarsch and Yanjing Wang.
Thomas Agotnes (U. Bergen)
"What does a group know? A new look at distributed knowledge"
ABSTRACT. In this talk I look at the traditional definition of distributed knowledge from two angles. Semantically, distributed knowledge is interpreted by taking the intersection of the accessability relations for the agents in the group. Said another way, the group considers a state impossible if at least one member of the group considers it impossible. We look at what happens when the "one agent" requirement is strengthened in different ways corresponding to different voting rules. We show that this notion of generalised distributed knowledge is a generalisation of both distributed knowledge and general knowledge (everybody-knows). This part of the talk is based on joint work with Dmitry Shkatov. Second, I will talk about reasoning about what happens when distributed knowledge is resolved. We introduce a new operator R_G for each group G. R_G phi means that phi is true after the group G has shared all their information with each other. This part of the talk is based on joint work with Yi Wang.
Thomas Bolander (DTU Copenhagen)
"False-belief tasks and their formalisation"
Andreas Herzig (CNRS, IRIT; joint work with Hans van Ditmarsch, Emiliano Lorini and Francois Schwarzentruber)
"Listen to me! Public announcements to agents that pay attention - or not"
ABSTRACT. In public announcement logic it is assumed that all agents pay attention (listen to/observe) to the announcement. Weaker observational conditions can be modelled in event (action) model logic. In this work, we propose a version of public announcement logic wherein it is encoded in the states of the epistemic model which agents pay attention to the announcement. This logic is called attention-based announcement logic, abbreviated ABAL. We give an axiomatization and prove that complexity of satisfiability is the same as that of public announcement logic, and therefore lower than that of action model logic. We exploit our logic to formalize the concept of joint attention that has been widely discussed in the philosophical and cognitive science literature. Finally, we extend our logic by integrating attention change.
Laurent Perrussel (IRIT; joint work with Guifei Jiang and Emiliano Lorini)
"Multi-agent Epistemic Explanatory Diagnosis via Reasoning about Actions"
ABSTRACT. We propose a modal logic that supports reasoning about trust-based belief change. The term *trust-based belief change* refers to belief change that depends on the degree of trust the receiver has in the source of information. On the technical level, our approach consists in extending Liau's static modal logic of belief and trust in three different directions: