*Modal logic* is a type of logic that includes modals—words or operators that qualify a statement in certain ways, such as expressing degrees of possibility or truth. The history of modal logic begins with Aristotle, who discussed it in chapters 12 and 13 of De Interpretatione, where he used modal logic to study relationships between the necessary, the impossible, and the possible.

For a modern discussion of modern logic, introducing notation may be helpful. the notation ◻`A` means that `A` is necessary, and ◇`A` means that `A` is possible.

There are modern modal logics that deal with other modalities, such as temporal modalities ("it has always been the case that *p*," "it was the case that *p*," etc.) or epistemic modalities ("it is known that *p*", etc.)