Ideas from 'works' by C.I. Lewis [1935], by Theme Structure

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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

			Full Idea: C.I.Lewis constructed five axiomatic systems of modal logic, and named them S1 to S5. It turns out that the simplest of the logics based on possible worlds is the same as Lewis's S5.
			From: report of C.I. Lewis (works [1935]) by Rod Girle - Modal Logics and Philosophy 2.1
			A reaction: Nathan Salmon ('Reference and Essence' 2nd ed) claims (on p.xvii) that "the correct modal logic is weaker than S5 and weaker even than S4". Which is the greater virtue, simplicity or weakness?

10. Modality / A. Necessity / 2. Nature of Necessity

11002

Equating necessity with informal provability is the S4 conception of necessity

			Full Idea: C.I.Lewis's S4 system develops a sense of necessity as 'provability' in some fairly informal sense.
			From: report of C.I. Lewis (works [1935]) by Stephen Read - Thinking About Logic Ch. 4