  Epistemic logic is another area in which logic in computer science have been influenced by philosophical logic. The classical source for epistemic logic is  Hintikka 1962,  in which Jaakko Hintikka showed that a modal approach to single-agent epistemic attitudes could be informative and rewarding. This work discusses at length the question of exactly which constraints are appropriate for knowledge and belief, when these attitudes are viewed as explicated by a model theoretic relation over possible worlds; in both cases, Hintikka argues for S4 type operators. 