07.05.2025 | Philosophische Fakultät, Philosophisches Seminar

The Additive Logic of Epistemic Reasons. An Axiomatic Account

In this talk Hannes Leitgeb will argue for a system of axioms that is meant to capture the logic of normative reasons for belief. The system concerns a primitive direct-reason relation, a defined doxastic-reason relation, an identity criterion for reasons, a primitive aggregation function for reasons, and a primitive function for the revision of belief by reasons. Reasons are assumed to speak for propositions with numerical strengths, the aggregation of reasons involves conjunctions of propositions and sums of strengths, belief is reconstructed as subjective probability, and ratios of new-to-old-odds are postulated to be a function of strengths of reason. The resulting theory avoids problems that have been ascribed to the additive aggregation of reasons, and it entails that reasons conform to a vector structure and that the impact of a reason on belief corresponds to a well-known probabilistic update method. The upshot is a systematic bridge between the philosophical theory of epistemic reasons and Bayesian formal epistemology.