On Thursday, the 22nd of January 2026, 17.00, Central European Time
Hannes Leitgeb
(Ludwig Maximilian University of Munich)
will deliver a talk:
The Additive Logic of Epistemic Reasons. An Axiomatic Account
Abstract:
In my talk I 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, and a primitive function for the revision of rational belief by reasons. Reasons are assumed to be facts that speak for belief with numerical strength, and the aggregation of reasons involves the intersection of sets of possible worlds and the summing up of strengths. Rational belief is reconstructed as subjective probability, and ratios of new-to-old odds are postulated to be a function of the strengths of reasons. The resulting theory avoids problems that have been ascribed to the additive aggregation of reasons; it entails that reasons exert epistemic forces that conform to a vector-like structure and that the rational impact of a reason on rational belief corresponds to a probabilistic Jeffrey/Field update. And although the theory differs from existing logics of reason, it is partially continuous with two of them. Its upshot is a systematic bridge between the philosophical theory of epistemic reasons and Bayesian formal epistemology.