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:
“You have promise, Mlle. Dubois, but you must choose between an operatic career and what is usually called “a normal life.” Though why it is so called is beyond me.”
—Eric Taylor, Leroux, and Arthur Lubin. M. Villeneuve (Frank Puglia)
“seizing the swift logic of a woman,
Curse God and die.”
—Edwin Arlington Robinson (1869–1935)