and it is closed under:
- Detachment rule (modus ponens): implies ;
- 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 normal (and hence 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.
Common normal modal logics
The following table lists several common normal modal systems. The notation refers to the table at Kripke semantics § Common modal axiom schemata. Frame conditions for some of the systems were simplified: the logics are sound and complete with respect to the frame classes given in the table, but they may correspond to a larger class of frames.
|S5||T, 5 or D, B, 4||equivalence relation|
|S4.3||T, 4, H||total preorder|
|S4.1||T, 4, M||preorder,|
|S4.2||T, 4, G||directed preorder|
|GL, K4W||GL or 4, GL||finite strict partial order|
|Grz, S4Grz||Grz or T, 4, Grz||finite partial order|
|D45||D, 4, 5||transitive, serial, and Euclidean|
- Alexander Chagrov and Michael Zakharyaschev, Modal Logic, vol. 35 of Oxford Logic Guides, Oxford University Press, 1997.