Master's Dissertation: A Semantics of (Justified) Evidence-Based Belief and Multi-Agent Notions Thereof

In this thesis, I present neighbourhood semantics as a framework for modelling evidence-based belief, as first presented in Van Benthem and Pacuit (2011). I argue why this is a well-suited framework for modelling this particular doxastic attitude, by postulating a number of desiderata that a good formal representation of evidence and evidence-based belief should satisfy, and by showing that neighbourhood semantics outperforms its competitors on almost all of these desiderata. I, further- more, explain and accommodate its shortcomings (i.e. the desideratum it does not fully satisfy) by making a qualification on the framework - I incorporate degrees of trustworthiness of the evidence a belief is based on into the models, to render the belief a justified belief, since for a belief to be justified, it being based on evidence is not sufficient, the evidence needs to, in addition, be trustworthy, i.e. come from a reliable source. After presenting the adapted semantics, I develop an epistemic hierarchy of different types of beliefs, and, I, furthermore, explain how the notion of trustworthiness of evidence I introduce differs from Van Benthem and Pacuit (2011)’s idea of reliable evidence. Lastly, I present and expand on Van Benthem and Pacuit (2011)’s suggestion for group notions of evidence and belief. I present their original notions of group evi- dence and formulate corresponding notions of group belief. In addition, I formulate clauses of group evidence and belief incorporating the notion of trustworthiness, and finally, present group notions of evidence and belief corresponding to the most prominent aggregation rules from the judgment aggregation literature.