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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. |