Independent agents in branching time : towards a unified framework for reasoning about multiagent systems
Nicolas Troquard · OpenGrey (Institut de l'Information Scientifique et Technique) · 2007
The work presented in this thesis is a multidisciplinary study of the notion of agency. We build new formal approaches starting on the literature of agency in philosophy of action, game theory or computer science. Belnap and Perloff's STIT theory is our frame of experimentation. This is a logic that stems from philosophy of action based on the observation that an action can be identi_ed with what it brings about. In this tradition, the sentence. Ishmael sails on board the Pequod. Will be paraphrased by. Ishmael sees to it that Ishmael sails on board the Pequod. . Our first contribution is to simplify the axiomatics of a version of the logic restrained to individual agency and without temporal aspects. This allows us to simplify the semantics of STIT as well as to discover a link with product logics. We establish the NEXPTIME-completeness of the problem of satisfiability. We capitalize on the simplifications and extend the axiomatization to coalitional actions. We show that we can embed Coalition Logic in the resulting logic. We also provide an epistemic extension and use it to tackle the problem of epistemically uniform strategies. Then we study further the temporal aspects of agency. We first do it by way of a logic combining STIT with a dynamic logic providing actions with duration, that can be deliberatively continued or aborted along time. We then give an embedding of Alternating-time Temporal Logic in a slightly adapted strategic STIT logic. Having developed a neat understanding of relevant structures of agency, we propose a fine-grained ontology of action and agency.