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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / g. System S4

[version imposing two conditions on accessibility]

7 ideas
If what is actual might have been impossible, we need S4 modal logic [Armstrong, by Lewis]
S4 has 14 modalities, and always reduces to a maximum of three modal operators [Cresswell]
In S4 the actual world has a special place [Dummett]
What is necessary is not always necessarily necessary, so S4 is fallacious [Salmon,N]
The system S4 has the 'reflexive' and 'transitive' conditions on its accessibility relation [Fitting/Mendelsohn]
There are seven modalities in S4, each with its negation [Girle]
S4 says there must be some necessary truths (the actual ones, of which there is at least one) [Cameron]