, first, that vague and fleeting images, especially of the
kinesthetic sort, are often present without being detected except by
very fine introspection, some image being pretty sure to come up every
few seconds when we are engaged in silent thought or {375} recall;
but, second, that images are not present every second of the time, and
that at the instant when a non-sensory fact is recalled it is apt to
be alone.

Hallucinations

Since a vivid mental image may be "in all respects the same as an
actual sensation", according to the testimony of some people, the
question arises how, then, an image is distinguished from a sensation.
Well, the image does not usually fit into the objective situation
present to the senses. But if it does fit, or if the objective
situation is lost track of, then, as a matter of fact, the image may
be taken for a sensation.

You see some beautiful roses in the florist's window, and you _smell_
them; the odor fits into the objective situation very well, till you
notice that the shop door is shut and the window glass impervious to
odors, from which you conclude that the odor must have been your
image.

You are lost in thought of an absent person, till, forgetting where
you are, you seem to see him entering the door; he "fits" well enough
for an instant, but then the present situation forces itself upon you
and the image takes its proper place.

You are half asleep, almost lost to the world, and some scene comes
before you so vividly as to seem real till its oddity wakens you to
the reality of your bedroom. Or you are fully asleep, and then the
images that come are dreams and seem entirely real, since contact with
the objective situation has been broken.

Images taken for real things are common in some forms of mental
disorder. Here the subject's hold on objective fact is weakened by his
absorption in his own desires and fears, and he hears reviling voices
and smells suspicious {376} odors or sees visions that are in line
with his desires and fears.

Such false sensations are called "hallucinations". An hallucination is
an image taken for a sensation, a recalled fact taken for a present
objective fact. It is a sensory response, aroused by a substitute
stimulus, without the subject's noticing that it is thus aroused
instead of by its regular peripheral stimulus.

Synesthesia.

Quite a large number of people are so constituted as to hear sounds as
if colored, a deep tone perhaps seemi