| 15544 | If what is actual might have been impossible, we need S4 modal logic [Armstrong, by Lewis] |
| 14972 | S4 has 14 modalities, and always reduces to a maximum of three modal operators [Cresswell] |
| 16958 | In S4 the actual world has a special place [Dummett] |
| 14671 | What is necessary is not always necessarily necessary, so S4 is fallacious [Salmon,N] |
| 9747 | The system S4 has the 'reflexive' and 'transitive' conditions on its accessibility relation [Fitting/Mendelsohn] |
| 7794 | There are seven modalities in S4, each with its negation [Girle] |
| 15102 | S4 says there must be some necessary truths (the actual ones, of which there is at least one) [Cameron] |