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:
and it is closed under:
- Detachment rule (Modus Ponens): ;
- Necessitation rule: implies .
The smallest logic satisfying the above conditions is called K. Most modal logics commonly used nowadays (in terms of having philosophical motivations), e.g. C. I. Lewis's S4 and S5, are extensions of K. However a number of deontic and epistemic logics, for example, are non-normal, often because they give up the Kripke schema.
Famous quotes containing the words normal and/or logic:
“Freedom is poetry, taking liberties with words, breaking the rules of normal speech, violating common sense. Freedom is violence.”
—Norman O. Brown (b. 1913)
“There is no morality by instinct.... There is no social salvation—in the end—without taking thought; without mastery of logic and application of logic to human experience.”
—Katharine Fullerton Gerould (1879–1944)