hallucination, begotten _from without_, that is by a vice in an
external organ, the eye. In case three, we have the origin of an
abnormal perception of something _unreal_, a hallucination, begotten
by a vicious activity _within_, in the sensory centre. But, while
all these three sets of stimuli set the machinery in motion, it is
the 'highest parts of the brain' that, in response to the stimuli,
create the full perception, real or hallucinatory.

But there remains a fourth way of setting the machinery in motion.
The first way, in normal sensation and perception, was the natural
action of the organ of sense, stimulated by a material object. The
second way was by the stimulus of a vice in the organ of sense. The
third way was a vicious activity in a sensory centre. All three
stimuli reach the 'central terminus' of the brain, and are there
created into perceptions, the first real and normal, the second a
hallucination from an organ of sense, _from without_, the third a
hallucination from a sensory centre, _from within_. The fourth way
is illustrated when the machinery is set a-going from the 'central
terminus' itself, 'from the higher parts of the brain, from the
seats of ideation and memory'. Now, as long as these parts only
produce and retain ideas or memories in the usual way, we think, or
we remember, but we have no hallucination. But when the activity
starting from the central terminus 'escapes downwards,' in
sufficient force, it reaches the 'lower centre' and the organ of
sense, and then the idea, or memory, stands visibly before us as a
hallucination.

This, omitting many technical details, and much that is matter of
more dispute than common, is a statement, rough, and as popular as
possible, of the ideas expressed in Mr. Gurney's remarkable essay on
hallucinations. {186} Here, then, we have a rude working notion of
various ways in which hallucinations may be produced. But there are
many degrees in being hallucinated, or enphantosme, as the old
French has it. If we are interested in the most popular kind of
hallucinations, ghosts and wraiths, we first discard like Le Loyer,
the evidence of many kinds of witnesses, diversely but undeniably
hallucinated. A man whose eyes are so vicious as habitually to give
him false information is not accepted as a witness, nor a man whose
brain is drugged with alcohol, nor a man whose 'central terminus' is
abandoned to religious excitement, to remorse, to grief, to anxi