A dream itself is but a shadow. - Hamlet, ii, 2.

In the numerous scientific books written about dreams a classification has been generally adopted dividing them into presentative (or sensorial) dreams, originating in physical impressions made upon the dreamer, and representative (or psychic) dreams, which originate in his mental impressions and in the memories which float up from the reserves of the brain; but the distinction thus made seems to be only a partially true one, for dreams do not actually fall thus into two perfectly separate groups. It would probably be nearer the truth to say that whereas all our dreams are made up of mental impressions and woven out of memories, sensory impressions and vibrations, starting either from within the body or from outside it, serve often to evoke those memories and to call up the mental images that dreams are composed of.

The dream consciousness tries to account for every sensory impression that it feels, and in order to do this it invents a coherent and plausible explanation which it proceeds to weave into a dream story. It is always difficult to know to what extent the dream consciousness is stimulated in this way by sensory impressions, because, apart from the nature and occurrences of the dream itself, it is generally impossible when we wake up to tell what sensations or vibrations played a part in and influenced the dream consciousness while we slept; whether, for instance, during our sleep a little noise occurred which may have suggested an incident in the dream, or whether some slight sense of bodily discomfort such as undue warmth or cold may have altered its current. In most cases we can only make a vague guess at the nature of these sense impressions, for the dream mind transforms the sound or other slightly disturbing sensation into something wholly different from the original physical cause. A piece of wood falls lightly on the hearth while we sleep; the dream consciousness does not recognise the sound for what it actually is, but may imagine it to be the crash of an avalanche or transform it into the sound of far-off guns; a wasp buzzes against the window-pane and the dream consciousness turns it into the drone of an aeroplane, and will forthwith develop the dream round the incident that is thus suggested.

Parables and symbols are the language and the means of expression of the dream consciousness, but we are often unable to recognise or to interpret the meaning of the fable that has thus been constructed. As Mr. Havelock Ellis has expressed it - whatever the physical cause may have been that has started the dream consciousness into activity - "the internal and external stimuli which act upon sleeping consciousness are not a part of that consciousness, nor in any real sense its source or its cause. The ray of sunlight that falls on the dreamer, the falling off of his bed-clothes, the indigestible supper he ate last night - these things can no more 'account' for his dream than the postman's knock can account for the contents of the letter he delivers. Whatever the stimuli from the physical world that may knock at the door of dreaming consciousness, that consciousness is apart from them, and stimuli can only reach it by undergoing transformation. They must put off the character they wear as phenomena of the waking world, they must put on the character of phenomena of another world, the world of dreams".

The following is an example of a transformation of this kind:

I had been thinking intently for a long time before I slept of the anxieties of a friend who was engaged in a difficult task of nursing. A sudden spell of cold brought on some slight rheumatic pain during the night, of which I had not been conscious before I went to sleep. The pain came into my dream but was transferred to my friend. In it she became the patient - I was helping to nurse her, and my mind was concentrated on the problems and practical difficulties that she and I had lately discussed; the pain, which was really my own, being one of the symptoms of the illness that she was suffering from in the dream.

This sensation was clearly one of the elements that entered into the construction of this dream, but it was only one element, for the thoughts that had filled my mind during the day and had preoccupied me before I slept gave the dream its shape and suggested practically all its incidents. Both sensation and memory alike underwent a dream change - they were transformed - and assumed, as Mr. Ellis has explained, the character of phenomena of the world of dreams:

It is certain that many dreams are thus stimulated by sensory impressions, often probably very slight ones which are never wholly extinguished during sleep; and if we could retrace our dreams more perfectly we should frequently find that incidents composing them originated in such a way as has been described. But dreams are of infinite variety, and I am convinced that there are also many that owe their existence to the action of memory alone, and that they are carried through by the dream mind without there having necessarily been any external stimulus or sense impression to account for them.

Amongst dreams that we find most often quoted as being initiated by physical causes are our dreams of flight. We are told by most writers on dreams that these are certainly due to sense impressions, and it is generally assumed that the sense of flying or floating is caused by the fact that in sleep our feet are not resting on the ground, that they miss the accustomed sense of the earth's resistance, and that the subconscious mind in the exercise of its reasoning faculty comes to the conclusion that this unfamiliar absence of pressure signifies a condition in which the body must be floating in mid-air. This may be the true cause of many flying dreams, but as far as I can judge from the series of experiments which I have made, it is not a satisfactory explanation of my own dreams of flying. This particular type of dream was chosen as one to be watched and recorded, and from these experiments it seems to me that sense impressions do not influence these dreams nearly as certainly and powerfully as mental impressions influence them.