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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / a. Systems of modal logic

[issues concerning the varieties of modal logic]

6 ideas
Relative possibility one way may be impossible coming back, so it isn't symmetrical [Dummett]
If something is only possible relative to another possibility, the possibility relation is not transitive [Dummett]
Non-S5 can talk of contingent or necessary necessities [Stalnaker]
Modal logic is multiple systems, shown in the variety of accessibility relations between worlds [Jacquette]
Necessity is provability in S4, and true in all worlds in S5 [Read]
With possible worlds, S4 and S5 are sound and complete, but S1-S3 are not even sound [Rossberg]