The Interpretation of Dreams by Sigmund Freud
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.