Jacob Purhard

“That is to say: Toward the accomplishment of an aim which in wantonness of malignity would seem to partake of the insane, he will direct a cool judgement sagacious and sound. These men are true madmen, and of the most dangerous sort, for their lunacy is not continuous but occasional, evoked by some special object; it is probably secretive, which is as much to say it is self-contained, so that when moreover, most active, it is to the average mind not distinguishable from sanity, and for the reason above suggested that whatever its aims may be--and the aim is never declared--the method and the outward proceeding are always perfectly rational.”
― Herman Melville, Billy Budd, Sailor