The discussion of the exciting causes of dreams takes up the most space in the descriptions of the authors. That this problem could appear only after the dream had become an object of biological investigation is self-evident. The ancients who conceived the dream as a divine inspiration had no need of looking for its exciting source; to them the dream resulted from the will of the divine or demoniacal powers, and its content was the product of their knowledge or intention. Science, however, soon raised the question whether the stimulus to the dream is always the same, or whether it might be manifold, and thus led to the question whether the causal explanation of the dream belongs to psychology or rather to physiology. Most authors seem to assume that the causes of the disturbance of sleep, and hence the sources of the dream, might be of various natures, and that physical as well as mental irritations might assume the rôle of dream inciters. Opinions differ greatly in preferring this or that one of the dream sources, in ranking them, and indeed as to their importance for the origin of dreams.
Wherever the enumeration of dream sources is complete we ultimately find four forms, which are also utilised for the division of dreams:—
I. External (objective) sensory stimuli. II. Internal (subjective) sensory stimuli. III. Internal (organic) physical excitations. IV. Purely psychical exciting sources.
I. The External Sensory Stimuli.—The younger Strümpell, son of the philosopher whose writings on the subject have already more than once served us as a guide in the problem of dreams, has, as is well known, reported his observations on a patient who was afflicted with general anæsthesia of the skin and with paralysis of several of the higher sensory organs. This man merged into sleep when his few remaining sensory paths from the outer world were shut off. When we wish to sleep we are wont to strive for a situation resembling the one in Strümpell’s experiment. We close the most important sensory paths, the eyes, and we endeavour to keep away from the other senses every stimulus and every change of the stimuli acting upon them. We then fall asleep, although we are never perfectly successful in our preparations. We can neither keep the stimuli away from the sensory organs altogether, nor can we fully extinguish the irritability of the sensory organs. That we may at any time be awakened by stronger stimuli should prove to us “that the mind has remained in constant communication with the material world even during sleep.” The sensory stimuli which reach us during sleep may easily become the source of dreams.
There are a great many stimuli of such nature, ranging from those that are unavoidable, being brought on by the sleeping state or at least occasionally induced by it, to the accidental waking stimuli which are adapted or calculated to put an end to sleep. Thus a strong light may force itself into the eyes, a noise may become perceptible, or some odoriferous matter may irritate the mucous membrane of the nose. In the spontaneous movements of sleep we may lay bare parts of the body and thus expose them to a sensation of cold, or through change of position we may produce sensations of pressure and touch. A fly may bite us, or a slight accident at night may simultaneously attack more than one sense. Observers have called attention to a whole series of dreams in which the stimulus verified on waking, and a part of the dream content corresponded to such a degree that the stimulus could be recognised as the source of the dream.
I shall here cite a number of such dreams collected by Jessen[[36]] (p. 527), traceable to more or less accidental objective sensory stimuli. “Every indistinctly perceived noise gives rise to corresponding dream pictures; the rolling of thunder takes us into the thick of battle, the crowing of a cock may be transformed into human shrieks of terror, and the creaking of a door may conjure up dreams of burglars breaking into the house. When one of our blankets slips off at night we may dream that we are walking about naked or falling into the water. If we lie diagonally across the bed with our feet extending beyond the edge, we may dream of standing on the brink of a terrifying precipice, or of falling from a steep height. Should our head accidentally get under the pillow we may then imagine a big rock hanging over us and about to crush us under its weight. Accumulation of semen produces voluptuous dreams, and local pain the idea of suffering ill treatment, of hostile attacks, or of accidental bodily injuries.”
“Meier (Versuch einer Erklärung des Nachtwandelns, Halle, 1758, p. 33), once dreamed of being assaulted by several persons who threw him flat on the ground and drove a stake into the ground between his big and second toes. While imagining this in his dream he suddenly awoke and felt a blade of straw sticking between his toes. The same author, according to Hemmings (Von den Traumen und Nachtwandeln, Weimar, 1784, p. 258) dreamed on another occasion that he was being hanged when his shirt was pinned somewhat tight around his neck. Hauffbauer dreamed in his youth of having fallen from a high wall and found upon waking that the bedstead had come apart, and that he had actually fallen to the floor.... Gregory relates that he once applied a hot-water bottle to his feet, and dreamed of taking a trip to the summit of Mount Ætna, where he found the heat on the ground almost unbearable. After having applied a blistering plaster to his head, a second man dreamed of being scalped by Indians; a third, whose shirt was damp, dreamed of being dragged through a stream. An attack of gout caused the patient to believe that he was in the hands of the Inquisition, and suffering pains of torture (Macnish).”
The argument based upon the resemblance between stimulus and dream content is reinforced if through a systematic induction of stimuli we succeed in producing dreams corresponding to the stimuli. According to Macnish such experiments have already been made by Giron de Buzareingues. “He left his knee exposed and dreamed of travelling in a mail coach at night. He remarked in this connection that travellers would well know how cold the knees become in a coach at night. Another time he left the back of his head uncovered, and dreamed of taking part in a religious ceremony in the open air. In the country where he lived it was customary to keep the head always covered except on such occasions.”
Maury[[48]] reports new observations on dreams produced in himself. (A number of other attempts produced no results.)
1. He was tickled with a feather on his lips and on the tip of his nose. He dreamed of awful torture, viz. that a mask of pitch was stuck to his face and then forcibly torn off, taking the skin with it.