 A formalisation of discussive logic is by means of modelling a discourse in a modal logic. For simplicity, Jaśkowski chose S5. We think of each participant’s belief set as the set of sentences true at a world in an S5 model \(M\). Thus, a sentence \(A\) asserted by a participant in a discourse is interpreted as “it is possible that \(A\)” or a sentence \(\Diamond A\) of S5. Then \(A\) holds in a discourse iff \(A\) is true at some world in \(M\). Since \(A\) may hold in one world but not in another, both \(A\) and \(\neg A\) may hold in a discourse. Indeed, one should expect that participants disagree on some issue in a rational discourse. The idea, then, is that \(B\) is a discussive consequence of \(A_1, \ldots, A_n\) iff \(\Diamond B\) is an S5 consequence of \(\Diamond A_{1} \ldots \Diamond A_{n}\).