in case of the accidental and external nerve stimuli, which may be recognised in the content of the dream without any trouble—nevertheless none of them has been able to avoid the admission that the abundant ideal content of dreams does not admit of explanation by external nerve-stimuli alone. Miss Mary Whiton Calkins12 has tested her own dreams and those of another person for a period of six weeks with this idea in mind, and has found only from 13.2 per cent. to 6.7 per cent. in which the element of external sensory perception was demonstrable; only two cases in the collection could be referred to organic sensations. Statistics here confirm what a hasty glance at our own experience might have led us to suspect.
The decision has been made repeatedly to distinguish the "dream of nerve stimulus" from the other forms of the dream as a well-established sub-species. Spitta64 divided dreams into dreams of nerve stimulus and association dreams. But the solution clearly remained unsatisfactory as long as the link between the somatic sources of dreams and their ideal content could not be demonstrated.
Besides the first objection, of the inadequate frequency of external exciting sources, there arises as a second objection the inadequate explanation of dreams offered by the introduction of this sort of dream sources. The representatives of the theory accordingly must explain two things, in the first place, why the external stimulus in the dream is never recognised according to its real nature, but is regularly mistaken for something else (cf. the alarm-clock dreams, p. 22), and secondly, why the reaction of the receiving mind to this misrecognised stimulus should result so indeterminately and changefully. As an answer to these questions, we have heard from Strümpell66 that the mind, as a result of its being turned away from the outer world during sleep, is not capable of giving correct interpretation to the objective sensory stimulus, but is forced to form illusions on the basis of the indefinite incitements from many directions. As expressed in his own words (p. 108):
"As soon as a sensation, a sensational complex, a feeling, or a psychic process in general, arises in the mind during sleep from an outer or inner nerve-stimulus, and is perceived by the mind, this process calls up sensory images, that is to say,