Found:

Modal logic

**SEMANTICS: **the logic of sentences with modal expressions like
*necessarily* and *possibly*. These expressions can be added as operators (say N and P) to standard logic:

(i) If phi is a formula, then Nphi is a formula (ii) If phi is a formula, then Pphi is a formulaSentences like these are interpreted in terms of possible worlds. Nphi is true if phi is true in every possible world; Pphi is true if phi is true in some possible world.

LIT. | Gamut, L.T.F. (1991) Hughes, G.E. and M.J. Cresswell (1968) |