of the mind to the stimulus attacking sleep is complicated and confused by the formation of illusions. A sensory impression is recognized by us and correctly interpreted- that is, it is classed with the memory- group to which it belongs according to all previous experience if the impression is strong, clear, and sufficiently prolonged, and if we have sufficient time to submit it to those mental processes. But if these conditions are not fulfilled we mistake the object which gives rise to the impression, and on the basis of this impression we construct an illusion. "If one takes a walk in an open field and perceives indistinctly a distant object, it may happen that one will at first take it for a horse." On closer inspection the image of a cow, resting, may obtrude itself, and the picture may finally resolve itself with certainty into a group of people sitting on the ground. The impressions which the mind receives during sleep from external stimuli are of a similarly indistinct nature; they give rise to illusions because the impression evokes a greater or lesser number of memory-images, through which it acquires its psychic value. As for the question, in which of the many possible spheres of memory the corresponding images are aroused, and which of the possible associative connections are brought into play, that- to quote Strumpell again- is indeterminable, and is left, as it were, to the caprices of the mind.

Here we may take our choice. We may admit that the laws of dream-formation cannot really be traced any further, and so refrain from asking whether or not the interpretation of the illusion evoked by the sensory impression depends upon still other conditions; or we may assume that the objective sensory stimulus encroaching upon sleep plays only a modest role as a dream- source, and that other factors determine the choice of the memory-image to be evoked. Indeed, on carefully examining Maury's experimentally produced dreams, which I have purposely cited in detail, one is inclined to object that his investigations trace the origin of only one element of the dreams, and that the rest of the dream-content seems too independent and too full of detail to be explained by a single requirement, namely, that it must correspond with the element experimentally introduced. Indeed, one even begins to doubt the illusion theory, and the power of objective impressions to shape the dream, when one realizes that such impressions are sometimes subjected to the most peculiar and far-fetched interpretations in our dreams. Thus M. Simon tells of a dream in which he saw persons of gigantic stature * seated at a table, and heard distinctly the horrible clattering produced by the impact of their jaws as they chewed their food. On waking he heard the clatter of a horse's hooves as it galloped past his window. If in this case the sound of the horse's hooves had revived ideas from the memory-sphere of Gulliver's Travels, the sojourn with the giants of Brobdingnag, and the virtuous horse-like creatures- as I should perhaps interpret the dream without any assistance on the author's part- ought not the choice of a memory-sphere so alien to the stimulus to be further elucidated by other motives?

* Gigantic persons in a dream justify the assumption that the dream is dealing with a scene from the dreamer's childhood. This interpretation of the dream as a reminiscence of Gulliver's Travels is, by the way, a good example of how an interpretation should not be made. The dream-interpreter should not permit his own intelligence to operate in disregard of the dreamer's impressions.

2. Internal (subjective) sensory stimuli

All objections to the contrary notwithstanding, we must admit that the role of the objective sensory stimuli as producers of dreams has been indisputably established, and if, having regard to their nature and their frequency, these stimuli seem perhaps insufficient to explain all dream- pictures, this indicates that we should look for other dream-sources which act in a similar fashion. I do not know where the idea first arose that together with the external sensory stimuli the internal (subjective) stimuli should also be considered, but as a matter of fact this has been done more or less explicitly in all the more recent descriptions of the aetiology of dreams. "I believe," says Wundt (p. 363), "that an important part is played in dream-illusions by those subjective sensations of sight and hearing which are familiar to us in the waking state as a luminous chaos in the dark field of the vision, and a ringing, buzzing, etc., of the ears, and in especial, subjective irritations of the retina. This explains the remarkable tendency of dreams to delude the eyes with numbers of similar or identical objects. Thus we see outspread before our eyes innumerable birds, butterflies, fishes, coloured beads, flowers, etc. Here the luminous dust in the dark field of vision has assumed fantastic forms, and the many luminous points of which it consists are embodied