dream-theories to be mentioned later, which seek to explain the absurdities and incoherences of dreams by a partial forgetting of what we have known during the day.

* That every impression, even the most insignificant, leaves an ineradicable mark, indefinitely capable of reappearing by day.

It might even occur to one to reduce the phenomenon of dreaming to that of remembering, and to regard the dream as the manifestation of a reproductive activity, unresting even at night, which is an end in itself. This would seem to be in agreement with statements such as those made by Pilcz, according to which definite relations between the time of dreaming and the contents of a dream may be demonstrated, inasmuch as the impressions reproduced by the dream in deep sleep belong to the remote past, while those reproduced towards morning are of recent origin. But such a conception is rendered improbable from the outset by the manner in which the dream deals with the material to be remembered. Strumpell rightly calls our attention to the fact that repetitions of experiences do not occur in dreams. It is true that a dream will make a beginning in that direction, but the next link is wanting; it appears in a different form, or is replaced by something entirely novel. The dream gives us only fragmentary reproductions; this is so far the rule that it permits of a theoretical generalization. Still, there are exceptions in which an episode is repeated in a dream as completely as it can be reproduced by our waking memory. Delboeuf relates of one of his university colleagues that a dream of his repeated, in all its details, a perilous drive in which he escaped accident as if by miracle. Miss Calkins mentions two dreams the contents of which exactly reproduced an experience of the previous day, and in a later chapter I shall have occasion to give an example that came to my knowledge of a childish experience which recurred unchanged in a dream. *

* From subsequent experience I am able to state that it is not at all rare to find in dreams reproductions of simple and unimportant occupations of everyday life, such as packing trunks, preparing food in the kitchen, etc., but in such dreams the dreamer himself emphasizes not the character of the recollection but its "reality"- "I really did this during the day."

C. Dream-Stimuli and Sources

What is meant by dream-stimuli and dream-sources may be explained by a reference to the popular saying: "Dreams come from the stomach." This notion covers a theory which conceives the dream as resulting from a disturbance of sleep. We should not have dreamed if some disturbing element had not come into play during our sleep, and the dream is the reaction against this disturbance.

The discussion of the exciting causes of dreams occupies a great deal of space in the literature of dreams. It is obvious that this problem could have made its appearance only after dreams had become an object of biological investigation. The ancients, who conceived of dreams as divine inspirations, had no need to look for stimuli; for them a dream was due to the will of divine or demonic powers, and its content was the product of their special knowledge and intention. Science, however, immediately raised the question whether the stimuli of dreams were single or multiple, and this in turn led to the consideration whether the causal explanation of dreams belonged to the region of psychology or to that of physiology. Most authors appear to assume that disturbance of sleep, and hence dreams, may arise from various causes, and that physical as well as mental stimuli may play the part of dream-excitants. Opinions differ widely in preferring this or the other factor as the cause of dreams, and in classifying them in the order of importance.

Whenever the sources of dreams are completely enumerated they fall into the following four categories, which have also been employed in the classification of dreams: (1) external (objective) sensory stimuli; (2) internal (subjective) sensory stimuli; (3) internal (organic) physical stimuli; (4) Purely psychical sources of excitation.