Welcome to the website of the DFG-GACR research project CELIA!

What we do:

The goal of the CELIA project is to study the consequences of lifting a significant, but until now, largely overlooked idealization in logical models of group knowledge, beliefs, and action: what we call the “extensional” view of groups. In most contributions to epistemic, doxastic, and coalition logic, a group is reduced to its extension, i.e., the set of its members. This has the immediate but counter-intuitive consequence that groups change identity when their membership changes, and rules out uncertainty regarding who is in a given group. This idealization does not reflect the structure of groups in the intended application of these logical models. Firms, informal teams, or even loose crowds typically remain even if they gain or lose members, and the identity of all members is rarely, if ever, common knowledge in any group of moderate size.

To lift this idealization, CELIA studies the “intensional” view of groups, which loosens the relationship between group membership and group identity, and applies it to questions in epistemic, doxastic, and coalition logic where collectives play a crucial role. CELIA is divided into three sub-projects. The first sub-project studies the logical properties of collective epistemic attitudes under the intensional view of groups by focusing, on the one hand, on specific algebraic or relational operations on groups and, on the other hand, on groups that are not logically omniscient. The second sub-project studies collective memory once intensional groups are taken into account. It studies, in particular, the relation between so-called individual and collective perfect recall, defined using distributed and common knowledge. The third sub-project re-visits coalition logic and studies groups’ agentive powers from an intensional perspective. In particular, it will consider how the canonical representation theorems for so-called playable effectivity functions change when intensional groups with specific algebraic structures are introduced.