Modal logic
= Modal logic
{wiki}
A <modal logic> extends propositional connectives by operators such as $\Box$ and its dual $\Diamond A=\neg\Box\neg A$. An interpretation determines their meaning: necessity, knowledge and formal provability lead to different axiom systems. In a relational semantics, $\Box A$ holds at a world when $A$ holds at every accessible world. <Provability logic> interprets the box through arithmetically coded proofs.