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.