On November 13, 2025, at 14:00 EET, Marian Călborean (University of Bucharest) will give a talk in the Logic Seminar.
Title: What we cannot say: An agent-relative, syntactic account of ignorance in epistemic logic
Abstract:
A theory of ignorance should distinguish uncertainty – lack of information and unawareness – lack of conception. As a new way to model incomplete epistemic descriptions, this paper introduces split languages, where each agent at each world reasons in a sublanguage that omits propositions and modalities for which there is no fact of the matter from that agent’s point of view. Standard Kripke frames extended with a map from agents and worlds to languages validate a language-relative notion of consequence and block inexpressible formulas at the syntax level. This yields transparent analyses of classical puzzles such as Ignorance, Mutual Knowledge, Muddy Children, and a coin underspecification puzzle, without assuming common knowledge of a fully specified model. In contrast to standard awareness logics, which stipulate an unstructured set of formulas an agent is aware of, the present proposal generates this set from a primitive vocabulary. This constraint provides a more principled account of unawareness, leading to a transparent, guarded proof system and a natural model of conceptual change. Thus, key meta-theoretic properties include conservativity over S5, soundness and completeness of the guarded proof system, and persistence under language growth. Finally, I introduce a dynamic operator for concept introduction and provide reduction principles.
The talk will take place physically at FMI (the new PBTower location), Hall 102.