The origin of the dream-image from physical sensations is conceived by Krauss as follows: The awakened sensation, in accordance with some law of association, evokes an idea or image bearing some relation to it, and combines with this idea or image, forming an organic structure, towards which, however, the consciousness does not maintain its normal attitude. For it does not bestow any attention on the sensation, but concerns itself entirely with the accompanying ideas; and this explains why the facts of the case have been so long misunderstood (p. 11 ff.). Krauss even gives this process the special name of "transubstantiation of the sensations into dream-images" (p. 24).

The influence of organic physical stimuli on the formation of dreams is today almost universally admitted, but the question as to the nature of the law underlying this relation is answered in various ways, and often obscurely. On the basis of the theory of physical excitation the special task of dream-interpretation is to trace back the content of a dream to the causative organic stimulus, and if we do not accept the rules of interpretation advanced by Scherner, we shall often find ourselves confronted by the awkward fact that the organic source of excitation reveals itself only in the content of the dream.

A certain agreement, however, appears in the interpretation of the various forms of dreams which have been designated as "typical," because they recur in so many persons with almost the same content. Among these are the well- known dreams of falling from a height, of the dropping out of teeth, of flying, and of embarrassment because one is naked or scantily clad. This last type of dream is said to be caused simply by the dreamer's perception, felt in his sleep, that he has thrown off the bedclothes and is uncovered. The dream that one's teeth are dropping out is explained by "dental irritation," which does not, however, of necessity imply a morbid condition of irritability in the teeth. According to Strumpell, the flying dream is the adequate image employed by the mind to interpret the quantum of stimulus emanating from the rising and sinking of the pulmonary lobes when the cutaneous sensation of the thorax has lapsed into insensibility. This latter condition causes the sensation which gives rise to images of hovering in the air. The dream of falling from a height is said to be due to the fact that an arm falls away from the body, or a flexed knee is suddenly extended, after unconsciousness of the sensation of cutaneous pressure has supervened, whereupon this sensation returns to consciousness, and the transition from unconsciousness to consciousness embodies itself psychically as a dream of falling (Strumpell, p. 118). The weakness of these fairly plausible attempts at explanation clearly lies in the fact that without any further elucidation they allow this or that group of organic sensations to disappear from psychic perception, or to obtrude themselves upon it, until the constellation favourable for the explanation has been established. Later on, however, I shall have occasion to return to the subject of typical dreams and their origin.

From a comparison of a series of similar dreams, M. Simon endeavoured to formulate certain rules governing the influence of organic sensations on the nature of the resulting dream. He says (p. 34): "If during sleep any organic apparatus, which normally participates in the expression of an affect, for any reason enters into the state of excitation to which it is usually aroused by the affect, the dream thus produced will contain representations which harmonize with that affect."

Another rule reads as follows (p. 35): "If, during sleep, an organic apparatus is in a state of activity, stimulation, or disturbance, the dream will present ideas which correspond with the nature of the organic function performed by that apparatus."

Mourly Vold has undertaken to prove the supposed influence of bodily sensation on the production of dreams by experimenting on a single physiological territory. He changed the positions of a sleeper's limbs, and compared the resulting dreams with these changes. He recorded the following results:

1. The position of a limb in a dream corresponds approximately to that of reality, i.e., we dream of a static condition of the limb which corresponds with the actual condition.

2. When one dreams of a moving limb it always happens that one of the positions occurring in the execution of this movement corresponds with the actual position.

3. The position of one's own limb may in the dream be attributed to another person.