Grant Morrison's graphic novel Arkham Asylum suggests that the Joker's mental state is in fact a previously unprecedented form of "super-sanity," a form of ultra-sensory perception. It also suggests that he has no true personality of his own, that on any given day he can be a harmless clown or a vicious killer, depending on which would benefit him the most. Later, during the Knightfall saga, after Scarecrow and the Joker team up and kidnap the mayor of Gotham City, Scarecrow turns on the Joker and uses his fear gas to see what Joker is afraid of. To Scarecrow's surprise, the gas has no effect on Joker, who in turn beats him with a chair. In Morrison's JLA title, the Martian Manhunter rewires his own brain in order to think like the Joker, and later briefly rewires the Joker's brain to create momentary sanity. In those few moments, the Joker realizes that he had to reevaluate his life and seemed to regret his various murders. He is returned to his usual self soon afterward.