(_b_) A _hallucination_ is a creation of the fancy mistaken for a reality. The deception may be but momentary, as when Macbeth is stealing on tiptoe to the chamber of his guest to murder him. His mind is disturbed by the imagination of the horrid deed he is about to perpetrate. He thinks he sees a dagger in the air, and he says: "Is this a dagger that I see before me, its handle towards my hand? Come, let me clutch thee. I hold thee not, and yet I see thee still; and on thy dudgeon gouts of blood, which was not so before." But Macbeth, upon a moment's reflection, sees it is all imagination. "There's no such thing," he exclaims. He is not insane, though deceived for a while. (_c_) A _delusion_, on the contrary, is a permanent deception, whether it results from an illusion or a hallucination, it matters not; as a fact, it almost always originates in hallucinations. The deluded man clings to his imaginings; you cannot talk them out of his head. Such is the case of an inebriate who suffers from _mania a potu_, or "the horrors;" he sees snakes and demons, he thinks, and persists in his error. Such also is a fixed idea not arrived at by faulty reasoning, but come unbidden and proof against all reasoning and evidence. Thus an insane man may be convinced, solely by his imagination, that he is poisoned or pursued or conspired against. 6. This delusion constitutes the essence of mental insanity, which therefore is often called delusional insanity. It may be chronic, i.e., of long continuance, or it may be temporary, acute. For the time being, the effects are the same. Perhaps any man may, at times, be for a moment thrown off his guard, and mistake a fancy for a reality; this does not constitute lunacy. But when the error is so firmly held in the mind's grasp that nothing can dislodge it thence, then the mind is deranged in its special sphere of action, which consists in knowing the real from the unreal; the mind is then insane.