We can often advance the study of a state which has hitherto been little known and examined, by comparing it with other states with whose symptoms we are better acquainted. We will, therefore, try to find points of correspondence with hypnosis. The very name shows that there is a resemblance between sleep (hypnos) and hypnosis, and some investigators (Liebeault, Bernheim, Brullard, Forel, Vires), consider hypnosis an ordinary sleep. They think that a person who falls asleep spontaneously is in rapport with himself, while a hypnotized subject is in rapport with the person who hypnotized him; in their view this is the chief difference between sleep and hypnosis. I believe, however, that we cannot so easily agree to such an identification of the states; we must begin by distinguishing the light and deep hypnoses.

We see that in light hypnosis there is merely an inhibition of voluntary movement; consciousness and self-consciousness are unaffected, and what happens during hypnosis is usually remembered. Now, in sleep there is always a great decrease of self-consciousness. But it is just this self-consciousness which remains intact in light hypnosis; and in this state the subject is perfectly aware of all that goes on, and, as a rule, forgets nothing on waking. Consequently, I do not think we can make a close connection between sleep and superficial hypnosis; nor do I think it possible to make a fruitful comparison between these light hypnoses and the states of drowsiness and fatigue which precede sleep. In any case, a feeling of fatigue is not uncommon in these hypnotic states. But this is not always the case, and we have seen that the loss of voluntary movement, or its subjection in a greater or less degree to the influence of the experimenter, is one of the chief phenomena in hypnosis. There is hardly a hint of this in the drowsy state; certainly there is a general heaviness in the limbs, but it is not of a nature to inhibit every voluntary movement; and the loss of voluntary movement in sleep is not subject to the influence of the experimenter as it is in hypnosis.

Further, these light hypnotic states are distinguished from the earlier stages of sleep by the decreased activity of consciousness in these latter. The current of ideas, of images of memory, is less under the control of the will; sense-impressions do not develop into conscious ideas in the usual way; much that generally excites our interest and attention is unnoticed and overlooked, while there is often reverie independent of will. But almost all this is entirely absent in the light hypnotic states, in which only the voluntary movements suffer change.

I cannot bring myself to consider a subject merely "asleep" so long as his consciousness and self-consciousness remain intact. There are, certainly, many investigators of a different opinion. Vogt, for example, even goes so far as to call anaesthesia induced by suggestion a state of partial sleep. According to him, all sleep depends upon some form of inhibition, which may set in and disappear with equal suddenness, may be lessened or terminated by a peripheral stimulus, but never attains the intensity of a real outbreak such as we meet with in cerebral disease. The anaesthesia induced by suggestion often presents these characteristics, and Vogt, therefore, calls it a state of partial sleep. Sollier holds similar views on the phenomena of hysteria. In his opinion, every hysterical person is more or less a somnambulist. For example, the somnambulic state in such cases is the result of anaesthesia, and Sollier calls the termination of the anaesthesia waking. According to him different regions of the brain participate in the production of sleep. But, as I have already mentioned, I do not think we are justified in associating such conditions with sleep, although in common parlance we speak of a limb being asleep.

Sollier's views, which are also held by some other investigators, may be accounted for to a certain extent by the fact that he and his adherents attribute all functional activity to definite cerebral processes. The holders of such views are consequently inclined to speak of partial sleep when certain portions of the body cease to functionate. But, from a psychological point of view, we must not speak of sleep unless there is some essential disturbance both of consciousness and self-consciousness.

The case of deep hypnosis is essentially different. It is characterized by sense-delusions which are just the same thing as our nightly dreams. In order to carry out the comparison, it will perhaps be well to consider the mode of origin of dreams in ordinary sleep. Dreams are divided into two classes, according to the manner of their origin (Spitta): (1) dreams induced by nerve-stimulation, and (2) dreams induced by association of ideas. The first - by far the most numerous - are induced by a peripheral stimulus of the nerves, affecting the brain. Here the stimulus is certainly felt; or a memory-image arises as well, and a perception results. This perception does not, however, correspond to the actual stimulus. What memory-image will be aroused, and what dream will result, depends upon many factors which for the most part escape our observation. Consequently, the memory-picture aroused by a stimulus attaches itself in a number of cases, but not invariably, to a previously existing dream. "When an orator dreams he is making a speech, he takes every noise for the applause of his imaginary hearers." Dreams which are called up by nerve stimulation often occur accidentally. Spitta relates that he once dreamed he was a gymnast.

In the dream he was thrown to the ground, bound, and boiling water poured on his left foot. The irritation thus produced was so unbearable that he awoke. His explanation is that he was very tired that night and forgot to take off his left sock, and the weather being very warm, the irritation set up by the sock caused the dream which frightened him. That dreams can be artifically called up by nerve-stimulation has been demonstrated by Gregory, MacNish, Maury, Leixner and many others. If a sleeping man is sprinkled with water he will dream of a shower of rain. Maury states that when Eau de Cologne was held to his nose he dreamed he was in Farina's shop at Cairo. Mourly Void put the limbs of certain of his patients in a fixed posture before they fell asleep, by bandaging their hands and feet together, and was thereby enabled to demonstrate that an enforced position influences the nature of the dream. This is in accord with an observation previously made by Child, that dream-activity is influenced by artificial changes in the muscular sense. I have taken these statements from the writings of Sante de Sanctis, who made many investigations into the question of artificially induced dreams.