In linguistics, a modal is an operator that is normally used
to indicate some mood distinct from indicative. Some modals in English are can, could, will, would,
shall, should, might, may, and must.
In philosophy, a modal is an operator usually related to possibility. The modal operators are box, which
usually means necessary, and diamond, which usually means possible. They are interdefinable as
follows: box P = not diamond not P; diamond P = not box not P. Computer scientists
have used box and diamond to represent other concepts than possibility, such as belief.
Modal logic derives in large part from C. I. Lewis, who developed various such logics;
probably the most commonly-used of these is S5.