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Ideas for 'fragments/reports', 'Alcibiades' and 'Thinking About Logic'

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

11004	Necessity is provability in S4, and true in all worlds in S5 [Read]
	Full Idea: In S4 necessity is said to be informal 'provability', and in S5 it is said to be 'true in every possible world'.
	From: Stephen Read (Thinking About Logic [1995], Ch.4)
	A reaction: It seems that the S4 version is proof-theoretic, and the S5 version is semantic.