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Ideas for 'works', 'A Theory of Universals' and 'works'

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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / g. System S4

15544

If what is actual might have been impossible, we need S4 modal logic [Armstrong, by Lewis]

4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / h. System S5

7791	The simplest of the logics based on possible worlds is Lewis's S5 [Lewis,CI, by Girle]