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Ideas for 'fragments/reports', 'Letters to Lelong' and 'A Theory of Universals'

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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]
	Full Idea: Armstrong says what is actual (namely a certain roster of universals) might have been impossible. Hence his modal logic is S4, without the 'Brouwersche Axiom'.
	From: report of David M. Armstrong (A Theory of Universals [1978]) by David Lewis - Armstrong on combinatorial possibility 'The demand'
	A reaction: So p would imply possibly-not-possibly-p.