 Fagin et al. 1984  begins a tradition in computational logic that revives the modal approach to epistemic logic, developing generalized logical foundations and applications that had not occurred to the philosophers. The technical idea is to simplify the modality, using S5 (or deontic S5 for belief), but to introduce multiple agents, and to concentrate on reasoning having to do with agents’ attitudes about one another’s attitudes. Such logics have direct applications in the analysis of distributed systems, dynamic systems in which change is effected by message actions, which change the knowledge of agents according to rules determined by a communications protocol.