Modal logic is an extension of predicate logic that includes concepts such as possibility, necessity, or impossibility. Thus, a modal language statement might be 'It is possible that it will rain today if and only if it is not possible that it will not rain today.' The contents of this book include The Method of Extension and Intension; L-Determinacy; The Method of the Name-Relation; On Metalanguages for Semantics; On the Logic of Modalities.
