There is a C axiom as mentioned at https://plato.stanford.edu/entries/logic-modal/#MapRelBetModLog in Section 8. My question is: what can be said about the formula which is the converse of the C axiom? I have been trying to find out whether the converse to C has been considered as an axiom or whether it can be formally proved in some modal axiom system.

  • I want to consider the converse to the C axiom in modal logics weaker than S5. – Monica Oct 4 '17 at 0:53

The converse of C,


is known as the McKinsey formula; it does not have a first-order frame condition.

It has also been discussed on Math SE.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.