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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. |