Many have endeavoured to explain hypnotism on physiological grounds. Cerebral anaemia, suspension of activity in the cerebral lobes, inhibition of the ganglion cells, and dissociation between nerve cells have all been advanced as possible theories to account for hypnosis. None, however, are completely satisfactory or of any practical importance.

Finally, there is the school of thought which regards hypnosis as a state of increased suggestibility. Suggestion is very powerful, even in the waking state, and everybody is suggestible—some more so than others.

A mere word or phrase can make a person feel happy, angry, ashamed, or afraid, and can often evoke the bodily symptoms or sensations which accompany these states of mind.

Consider how a single word or idea may cause the blood to rush to the face and so give rise to a blush. Our whole lives are constantly being influenced by suggestion. Yawning, for instance, is notoriously infectious; and if one person faints in a crowd, others will do likewise. Advertising depends upon the force of suggestion to induce people to buy things which they often do not really need The skill of the advertiser consists in making the people think they need what is offered for sale. Orators, agitators and Members of Parliament all use suggestion to sway public opinion and so persuade people to follow them. Certain circumstances increase suggestibility: complete relaxation, for instance. Everybody can see pictures in the fire while resting quietly. Again, strong emotion sensitizes the brain and renders a person more susceptible to suggestion. When emotion enters the picture, reason is relegated to the background. This explains why a crowd of people, for instance, can be incited to an act of violence, by an agitator, when they would never dream of doing such a thing in their ordinary state of mind.

Further, the imagination is much stronger than the will, and in any conflict between the two the imagination will always win. When a person is in a state of mind in which he would like to do a certain thing but thinks he cannot, then he will be unable to do it.

Limitation of the field of consciousness, and concentration of attention, also help to increase the force of suggestion. When suggestion is applied, scientifically, using some if not all of the factors mentioned, then it is possible to produce the state of hypersuggestibility which we know as hypnosis.

This does not mean that hypnosis is ‘simple suggestion’, as many people who would like to belittle the subject claim.

In ordinary suggestion in the waking state, thoughts and ideas are diluted with thousands of other thoughts. In hypnosis these extraneous thoughts are removed, and the mind is able to concentrate all its force on the suggestions of the hypnotist—hence suggestibility is greatly increased.

Even in the waking state an idea which is implanted in the mind by suggestion tends to be carried out unless it is definitely inhibited.

In hypnosis this inhibiting influence is usually entirely removed and the patient will carry out all orders which do not conflict with his deep-rooted moral principles.

For all practical purposes, then, it is best to regard hypnosis as a peculiar psychological state in which the mind is particularly susceptible to suggestion. This state may be brought about by various means, by either the hypnotist or the patient himself, in which case it is known as auto-hypnosis. This theory does not, of course, explain everything, but it does provide a practical working basis. In addition it is acceptable to the patient, who is usually able to grasp the importance of suggestion, and it effectively disabuses his mind of any ‘occult’ notions concerning the value of hypnosis. Further, it is possible, by using suggestion in the waking state, to demonstrate to the patient that words have power over the mind and body. There are certain well-known tests of suggestibility, which will be discussed later, that enable the hypnotist to judge whether a patient will be likely to make a good subject or not.

Many of these tests actually prepare the patient for hypnosis for, when he finds himself unable to perform some ordinary task simply as the result of suggestion, although he is wide awake, the mind is impressed and susceptibility to hypnosis is increased. Stage hypnotists are well aware of the value of these tests and (as will be described later) make full use of them to select easy subjects for their demonstrations.

Thus, while it is true that it is no real explanation to say that hypnosis is merely a state of exaggerated susceptibility produced by suggestion, from a practical point of view it is the most useful theory we have.

Application of the principles of suggestion enable us to choose with reasonable certainty those patients who will be likely to make good subjects. Further, it enables us to apply the appropriate methods to induce hypnosis according to the various needs of different patients. Finally, by suggestion we are able to evoke the various phenomena required and bring about the curative effects of hypnosis.

In the ordinary state the mind is occupied with many different thoughts, and suggestion given in this state literally goes ‘in one ear and out the other’. Only a small portion of the mind absorbs the suggestion, and so the effect is very weak.

When the patient is hypnotized, the mind is concentrated, so that more of the suggestion is absorbed, and so the effect is greater. After hypnosis, when the mind returns to its normal state, each ‘unit’ of the mind will carry a dose of suggestion, so that the patient will ‘think’ and ‘feel’ as suggested. One hundred per cent. concentration would, of course, represent the very deepest form of hypnosis, and in this condition it is easy to see why the patient could develop anaesthesia. 
If 100 per cent of the mind is concentrated on the hypnotist’s suggestions, then there is nothing left to take notice of pain.

Fortunately, for most medical purposes, 100 % concentration is not required. If only 51 %. of the mind can be affected by suggestion, it will outweigh the other 49 %. This explains why light hypnosis is quite sufficient for most medical purposes if the proper technique is employed.

This explanation would seem to show that hypnosis is really a concentration of the mind.