A Method of Generating Modal Logics Defining Jaśkowski’s Discussive Logic D2

Abstract

Jaśkowski's discussive logic D2 was formulated with the help of the modal logic S5 as follows (see [7, 8]): $${A \in {D_{2}}}$$ iff $${\ulcorner\diamond{{A}^{\bullet}}\urcorner \in {\rm S}5}$$ , where (---)? is a translation of discussive formulae from Ford into the modal language. We say that a modal logic L defines D2 iff $${{\rm D}_{2} = \{A \in {\rm For^{\rm d}} : \ulcorner\diamond{{A}^{\bullet}}\urcorner \in {\it L}\}}$$ . In [14] and [10] were respectively presented the weakest normal and the weakest regular logic which (?): have the same theses beginning with ` $${\diamond}$$ ' as S5. Of course, all logics fulfilling the above condition, define D2. In [10] it was prowed that in the cases of logics closed under congruence the following holds: defining D2 is equivalent to having the property (?). In this paper we show that this equivalence holds also for all modal logics which are closed under replacement of tautological equivalents (rte-logics). We give a general method which, for any class of modal logics determined by a set of joint axioms and rules, generates in the given class the weakest logic having the property (?). Thus, for the class of all modal logics we obtain the weakest modal logic which owns this property. On the other hand, applying the method to various classes of modal logics: rte-logics, congruential, monotonic, regular and normal, we obtain the weakest in a given class logic defining D2.