unken man sees two posts where there
is only one. He has a picture of the post in each eye, and his brain is
too much disturbed to refer the two pictures to the same object. In this
case the cause of the mistake is subjective. A _mirage_ offers another
instance of a sense-illusion; but in it the cause is objective.

(_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.

You notice, gentlemen, that I speak of the mind as grasping the error,
and I suppose it to do so independently of the free will's command. But
when the error is voluntary; when a man clin