To write strongly the history of our scientific knowledge of the dream- problem is extremely difficult, because, valuable though this knowledge may be in certain respects, no real progress in a definite direction is as yet discernible. No real foundation of verified results has hitherto been established on which future investigators might continue to build. Every new author approaches the same problems afresh, and from the very beginning. If I were to enumerate such authors in chronological order, giving a survey of the opinions which each has held concerning the problems of the dream, I should be quite unable to draw a clear and complete picture of the present state of our knowledge on the subject. I have therefore preferred to base my method of treatment on themes rather than on authors, and in attempting the solution of each problem of the dream I shall cite the material found in the literature of the subject.

But as I have not succeeded in mastering the whole of this literature- for it is widely dispersed, and interwoven with the literature of other subjects- I must ask my readers to rest content with my survey as it stands, provided that no fundamental fact or important point of view has been overlooked.

Until recently most authors have been inclined to deal with the subjects of sleep and dreams in conjunction, and together with these they have commonly dealt with analogous conditions of a psycho-pathological nature, and other dream-like phenomena, such as hallucinations, visions, etc. In recent works, on the other hand, there has been a tendency to keep more closely to the theme, and to consider, as a special subject, the separate problems of the dream-life. In this change I should like to perceive an expression of the growing conviction that enlightenment and agreement in such obscure matters may be attained only by a series of detailed investigations. Such a detailed investigation, and one of a special psychological nature, is expounded in these pages. I have had little occasion to concern myself with the problem of sleep, as this is essentially a physiological problem, although the changes in the functional determination of the psychic apparatus should be included in a description of the sleeping state. The literature of sleep will therefore not be considered here.

A scientific interest in the phenomena of dreams as such leads us to propound the following problems, which to a certain extent, interdependent, merge into one another.

A. The Relation of the Dream to the Waking State

The naive judgment of the dreamer on waking assumes that the dream- even if it does not come from another world- has at all events transported the dreamer into another world. The old physiologist, Burdach, to whom we are indebted for a careful and discriminating description of the phenomena of dreams, expressed this conviction in a frequently quoted passage (p. 474): "The waking life, with its trials and joys, its pleasures and pains, is never repeated; on the contrary, the dream aims at relieving us of these. Even when our whole mind is filled with one subject, when our hearts are rent by bitter grief, or when some task has been taxing our mental capacity to the utmost, the dream either gives us something entirely alien, or it selects for its combinations only a few elements of reality; or it merely enters into the key of our mood, and symbolizes reality." J. H. Fichte (I. 541) speaks in precisely the same sense of supplementary dreams, calling them one of the secret, self-healing benefits of the psyche. L. Strumpell expresses himself to the same effect in his Natur und Entstehung der Traume, a study which is deservedly held in high esteem. "He who dreams turns his back upon the world of waking consciousness" (p. 16); "In the dream the memory of the orderly content of waking consciousness and its normal behaviour is almost entirely lost" (p. 17); "The almost complete and unencumbered isolation of the psyche in the dream from the regular normal content and course of the waking state..." (p. 19).

Yet the overwhelming majority of writers on the subject have adopted the contrary view of the relation of the dream to waking life. Thus Haffner (p. 19): "To begin with, the dream continues the waking life. Our dreams always connect themselves with such ideas as have shortly before been present in our consciousness. Careful examination will nearly always detect a thread by which the dream has linked itself to the experiences of the previous day." Weygandt (p. 6) flatly contradicts the statement of Burdach. "For it may often be observed, apparently indeed in the great majority of dreams, that they lead us directly back into everyday life, instead of releasing us from it." Maury (p. 56) expresses the same idea in a concise