An Observation Concerning Porte’s Rule in Modal Logic
Rohan French, Lloyd Humberstone · Bulletin of the Section of Logic · 2015
It is well known that no consistent normal modal logic contains (as theorems) both ◊A and ◊¬A (for any formula A). Here we observe that this claim can be strengthened to the following: for any formula A, either no consistent normal modal logic contains ◊A, or else no consistent normal modal logic contains ◊¬A.