In logic, a normal modal logic is a set L of modal formulas such that L contains: All propositional tautologies; All instances of the Kripke schema: ◻ → {\displaystyle \Box \to } and it is closed under: Detachment rule : A → B, A ∈ L {\displaystyle A\to B,A\in L} implies B ∈ L {\displaystyle B\in L} ; Necessitation rule: A ∈ L {\displaystyle A\in L} implies ◻ A ∈ L {\displaystyle \Box A\in L}. Wikipedia