Abstract

The paper appreciates six conditions which, according to G.E. Hughes and M.J. Cresswell, must be respected by those formal systems which aspire to be systems of modal logic. There are few such systems. The sense of modal functors \"it is necessary that,\" \"it is possible that,\" in this type of systems is in no way made precise. The paper undermines the validity of some conditions shown by Hughes and Cresswell. It outlines the way by which to construct a system of modal logic in which the functor of logical necessity and logical possibility would be unambiguously characterised by methods of contemporary logic