In this course we will first learn the basics of set theory, which will give us a vocabulary with which to approach modal logic, our main topic. Modal logic is the study of relations among claims about what is possibly true or necessarily true. We interpret these claims as claims about possible worlds, among which we postulate an accessibility relation: some worlds can “see” others. The basic idea is that if from your world you can “see” a world at which proposition p is true then it is true, at your world, that it is possibly true that p. By postulating different accessibility relations one gets different modal logics; we will understand which sorts of relations generate which logics. Familiarity with the semantics of modal logic is presupposed in a good deal of contemporary philosophical research. Moreover it’s fun and imagination-stretching to consider “blind” worlds and “all-seeing” worlds.