Modal Logics in the Vicinity of S1
Brian F. Chellas, Krister Segerberg · Notre Dame Journal of Formal Logic · 1996
We define prenormal modal logics and show that S1, S1$^\circ$, S0.9, and S0.9$^\circ$ are Lewis versions of certain prenormal logics, determination and decidability for which are immediate. At the end we characterize Cresswell logics and ponder C. I. Lewis's idea of strict implication in S1.