Notes
S5 is the normal modal logic developed by C. I. Lewis and C. H. Langford in 1932. S5 holds that all modal truths are necessary truths. Thus, a proposition that would be necessary in some nonactual possible world is necessary in the actual world: ◊ □p → □p.
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Sula, C.A. Ralph Wedgwood, The Nature of Normativity . Ethic Theory Moral Prac 11, 227–228 (2008). https://doi.org/10.1007/s10677-007-9105-3
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DOI: https://doi.org/10.1007/s10677-007-9105-3