
"It is necessarily true that A" means that things being as they are, A must be true, e.g. "It is necessarily true that x=x" is TRUE while "It is necessarily true that x=y" is FALSE even though "x=y" might be TRUE. Adding modal operators [F] and [P], meaning, respectively, henceforth and hitherto leads to a "temporal logic". Flavours of modal logics include: {Propositional Dynamic Logic} (PDL), {Propositional Linear Temporal Logic} (PLTL), {Linear Temporal Logic} (LTL), {Computational Tree Logic} (CTL), {HennessyMilner Logic}, S1S5, T. C.I. Lewis, "A Survey of Symbolic Logic", 1918, initiated the modern analysis of modality. He developed the logical systems S1S5. JCC McKinsey used algebraic methods ({Boolean algebras スポンサード リンク
