The Additive Logic of Epistemic Reasons. An Axiomatic Account

Hannes Leitgeb (LMU Munich, LMU Munich)

The next meeting of the Sign-Language-Reality seminar in the academic year 2025/26 will take place:  

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.

About the speaker:

Hannes Leitgeb is an Austrian philosopher and mathematician, and one of the central figures in contemporary logic and formal epistemology. He is Professor of Philosophy, Alexander von Humboldt Professor, and Chair of Logic and Philosophy of Language at Ludwig Maximilian University of Munich, where he also co-directs the Munich Center for Mathematical Philosophy. His work ranges widely across logic (truth, modality, paradoxes, conditionals, nonmonotonic reasoning), epistemology (belief, probability, Bayesianism, belief revision), philosophy of mathematics, philosophy of language, philosophy of science, and cognitive science.

Leitgeb is the author of The Stability of Belief: How Rational Belief Coheres with Probability (Oxford University Press, 2017), and co-editor of volumes such as Logic, Methodology and Philosophy of Science: Proceedings of the Fifteenth International Congress and Reduction – Abstraction – Analysis. He has also published many papers that have shaped current debates about truth, probability, and rational belief.

A further strand of his research concerns meaning similarity and semantic resemblance. In this area he has developed precise formal frameworks for similarity of contents and proved a striking impossibility result showing that certain natural principles about meaning similarity and compositionality cannot all be satisfied at once.

Leitgeb’s contributions have been recognised with numerous honours, including membership in the German National Academy of Sciences Leopoldina and the Academia Europaea, as well as major research prizes such as the Gottfried Wilhelm Leibniz Prize and the Frege Prize.

The seminar will be held online, to join the meeting, please use the information below:

Join Zoom Meeting  https://uw-edu-pl.zoom.us/j/92716044372?pwd=0l7PETAOwqQDBKTMCnheYQN7ag7zx1.1

Meeting ID: 927 1604 4372 Passcode: 697648   The meeting opens at 4:45 pm, the talk starts at 5 pm.   

https://www.pts.edu.pl/seminarium