Floor Sietsma

On the logic of lying

We model lying as a communicative act changing the beliefs of the agents in a multi-agent system. With Augustine, we see lying as an utterance believed to be false by the speaker and uttered with the intent to deceive the addressee. The deceit is successful if the lie is believed after the utterance by the addressee. This is our perspective. Also, as common in dynamic epistemic logics, we model the agents addressed by the lie, but we do not (necessarily) model the speaker as one of those agents. This further simplifies the picture: we do not need to model the intention of the speaker, nor do we need to distinguish between knowledge and belief of the speaker: he is the observer of the system and his beliefs are taken to be the truth by the listeners. We provide a sketch of what goes on logically when a lie is communicated. We present a complete logic of manipulative updating, to analyse the effects of lying in public discourse. Next, we turn to the study of lying in games. First, a game-theoretical analysis is used to explain how the possibility of lying makes games such as Liars Dice interesting, and how lying is put to use in optimal strategies for playing the game. This is the opposite of the logical manipulative update: instead of always believing the utterance, now, it is never believed. We also give a matching logical analysis for the games perspective, and implement that in the model checker DEMO. Our running example of lying in games is the game of Liars Dice.

Modelling Cryptographic Keys in Dynamic Epistemic Logic with DEMO

It is far from obvious to find logical counterparts to crytographic protocol primitives. In logic, a common assumption is that agents are perfectly rational and have no computational limitations. This creates a dilemma. If one merely abstracts from computational aspects, protocols become trivial and the difference between tractable and intractable computation, surely an essential feature of protocols, disappears. This way, the protocol gets lost. On the other hand, if one ‘merely′ (scare quotes indeed) models agents with computational limitations (or otherwise bounded rationality), very obvious aspects of reasoning become problematic. That way, the logic gets lost. We present a novel way out of this dilemma.We propose an abstract logical architecture wherein public and private, or symmetric keys, and their roles in crytographic protocols, all have formal counterparts. Instead of having encryption and decryption done by a principal, the agent sending or receiving messages, we introduce additional, virtual, agents to model that, so that one-way-function aspects of computation can be modelled as constraints on the communication between principals and these virtual counterparts. In this modelling it does not affect essential protocol features if agents are computationally unlimited.We have implemented the proposal in a dynamic epistemic model checker called DEMO.

Message passing in a dynamic epistemic logic setting

We propose a framework for message passing that combines the best properties of dynamic epistemic semantics and history-based approaches. We assume that all communication is truthful and reliable. Our framework consists of Kripke models with records of sent messages in their valuations. We introduce an update operation for message sending. With this update we can study the exact epistemic consequences of sending a message. We define a class of models that is generated from initial Kripke models by means of message updates, and we axiomatize a logic for this class of models. Next, we add an update modality and sketch a procedure for defining it by means of equivalence axioms.

Logic of information flow on communication channels

We develop an epistemic logic to specify and reason about information flow and its underlying communication channels. By combining ideas from Dynamic Epistemic Logic (DEL) and Interpreted Systems (IS), our semantics offers a natural and neat way of modelling multi-agent communication scenarios with different assumptions about the observational power of agents.

Common Knowledge in Email Exchanges

We consider a framework in which a group of agents communicates by means of emails, with the possibility of replies, forwards and blind carbon copies (BCC). We study the epistemic consequences of such email exchanges by introducing an appropriate epistemic language and semantics. This allows us to find out what agents learn from the emails they receive and to determine when a group of agents acquires common knowledge of the fact that an email was sent. We also show that in our framework from the epistemic point of view the BCC feature of emails cannot be simulated using messages without BCC recipients.

Multi-agent belief revision with linked preferences

In this paper we forge a connection between dynamic epistemic logics of belief revision on one hand and studies of collective judgement and multi-agent preference change on the other. Belief revision in the spirit of dynamic epistemic logic uses updating with relational substitutions to change the beliefs of individual agents. Collective judgement in social choice theory studies the collective outcomes of individual belief changes. We start out from the logic of communication and change (LCC) without constraints, and then study the effects of imposing a single constraint, namely the constraint that the agents preference relations are linked. Finally, we show that the resulting framework can be used to model consensus seeking procedures. We focus on the case of plenary Dutch meetings. In Dutch meetings, a belief change (or rather: preference change) is performed for all agents in the meeting if a majority believes (or: is in favour of) the proposition that is under discussion. A special case of these meetings is judgement aggregation, and we apply our framework to the discursive dilemma in this field. Our framework has obvious connections to coalition logic and social choice theory.

Reflections on vote manipulation

The notion of non-manipulability (or: strategy-proofness) used in the famous Gibbard-Satterthwaite theorem is too strong to make useful distinctions between voting rules.We explore alternative definitions and suggest how these can be used to classify voting rules.