This philosophy course is listed as a possible elective for my major, but I don't have a clue about this description. First course in two-term sequence (also see course 176). Topics include various normal modal systems, derivability within the systems, Kripke-style semantics and generalizations, Lemmon/Scott completeness, incompleteness in tense and modal logic, quantificational extensions. Can anyone tell me briefly what a modal system is? Thanks!