Did Medieval Logicians know the S5 Lewis Modal System?An answer from the Medieval Modal Octagon
Abstract
We address in this essay the medieval modal octagon and find that two sentences there admit simple conversion, which is expressed as equivalence. Those equivalences can be proved in the S5 Lewis Modal System. We begin by showing the relevance of S5, then we show typical theorems of different modal logics (K, T, S4, B and S5) and then we address the medieval argumentation. This argument requires to be formulated in S5, which allows us to give an, in some sense, affirmative answer to our initial question.References
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