'Knowable' as 'known after an announcement'
Résumé
Public announcement logic is an extension of multi-agent epistemic logic withdynamic operators to model the informational consequencesof announce-ments to the entire group of agents. We propose an extension of public an-nouncement logic with a dynamic modal operator that expresses what is trueafteranyannouncement:♦φexpresses that there is a truthful announcementψafter whichφis true. This logic gives a perspective on Fitch’s knowabilityissues: for which formulasφdoes it hold thatφ→♦Kφ? We give various se-mantic results, and we show completeness for a Hilbert-style axiomatizationof this logic. There is a natural generalization to a logic for arbitrary events.