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Ideas for 'works', 'Beitrage' and 'Intro: Theories of Vagueness'

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

9065	S5 collapses iterated modalities (◊□P→□P, and ◊◊P→◊P) [Keefe/Smith]
	Full Idea: S5 collapses iterated modalities (so ◊□P → □P, and ◊◊P → ◊P).
	From: R Keefe / P Smith (Intro: Theories of Vagueness [1997], §5)
	A reaction: It is obvious why this might be controversial, and there seems to be a general preference for S4. There may be confusions of epistemic and ontic (and even semantic?) possibilities within a single string of modalities.