If the conscious interest, together with the inner and outer sleep stimuli, sufficed to cover the etiology of the dreams, we ought to be in a position to give a satisfactory account of the origin of all the elements of a dream; the riddle of the dream sources would thus be solved, leaving only the task of separating the part played by the psychic and the somatic dream stimuli in individual dreams. But as a matter of fact no such complete solution of a dream has ever been accomplished in any case, and, what is more, every one attempting such solution has found that in most cases there have remained a great many components of the dream, the source of which he was unable to explain. The daily interest as a psychic source of dreams is evidently not far-reaching enough to justify the confident assertions to the effect that we all continue our waking affairs in the dream. Other psychic sources of dreams are unknown. Hence, with the exception perhaps of the explanation of dreams given by Scherner,[58] which will be referred to later, all explanations found in the literature show a large gap when we come to the derivation of the material for the presentation pictures, which is most characteristic for the dream. In this dilemma the majority of authors have developed a tendency to depreciate as much as possible the psychic factor in the excitations of dreams which is so difficult to approach. To be sure, they distinguish as a main division of dreams the nerve-exciting and the association dreams, and assert that the latter has its source exclusively in reproduction (Wundt,[76] p. 365), but they cannot yet dismiss the doubt whether “they do not appear without being impelled by the psychical stimulus” (Volkelt,[72] p. 127). The characteristic quality of the pure association dream is also found wanting. To quote Volkelt (p. 118): “In the association dreams proper we can no longer speak of such a firm nucleus. Here the loose grouping penetrates also into the centre of the dream. The ideation which is already set free from reason and intellect is here no longer held together by the more important psychical and mental stimuli, but is left to its own aimless shifting and complete confusion.” Wundt, too, attempts to depreciate the psychic factor in the stimulation of dreams by declaring that the “phantasms of the dream certainly are unjustly regarded as pure hallucinations, and that probably most dream presentations are really illusions, inasmuch as they emanate from slight sensory impressions which are never extinguished during sleep” (p. 338, &c.). Weygandt[75] agrees with this view, but generalises it. He asserts that “the first source of all dream presentations is a sensory stimulus to which reproductive associations are then joined” (p. 17). Tissié[68] goes still further in repressing the psychic exciting sources (p. 183): “Les rêves d’origine absolument psychique n’existent pas”; and elsewhere (p. 6), “Les pensées de nos rêves nous viennent de dehors....”