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 seeming dark blue, the sound of a trumpet a vivid red, etc. Each vowel and even each consonant may have its own special color, which combine to give a complex color scheme for a word. Numbers also may be colored. This colored hearing is the commonest form of "synesthesia", which consists in responding to a stimulus acting on one sense, by sensations belonging to a different sense. Whether the persons so constituted as to respond in this way are constituted thus by nature or by experience is uncertain, though the best guess is that the extra sensations are images that have become firmly attached to their substitute stimuli during early childhood.