In a previous post, I introduced modal logic as a common way of adding an operation from propositions to propositions to ordinary logic. In that post, I noted that you want to add the operator, and then axiomatize some properties of it. Now I want to examine the model of modal logic, to shed more light on what the axioms mean. The particular type of model used for modal logic is called a Kripke structure.1 [1 More precisely, only “normal” modal logics have a Kripke structure as their sema...
