In modal logic, □ is the necessity operator.1

It is necessary that P.

For P to be necessary is for P to be true in all possible worlds. Compare □P with the modal sentence ◊P, which means P is true only in some possible world (possibly P).

□ can be formally defined using , which is the modal possibility operator, and the negation operator ¬ from classical logic.

□P = ¬◊¬P
Necessarily P = Not possibly not P
1For those who are dubious of their browser's character set, □ is indeed meant to display as a box.

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