@exploringlove: S4 is so underrated n overlooked it can be S5 (Zombie-S5) and function like one without symmetric n the AxAdmissibility. Box n diamonds are monad-commonad, and yes Kripke Completeness theorem only proved the Equivalence in S5 not that S5 is independent n a upgrade ver of s4, s5 is a reification of s4. it works better in theoretic spaces, beyond alexandrov spaces where s5 is restricted to. #philosophy #logic