The modern endeavour to pursue Psychology, as all the physical sciences are now pursued, by the study of facts and phenomena, instead of by metaphysical abstractions, consulting of inner consciousness and argument à priori, has invested the subject of this monograph with extraordinary importance, because Sleep and Dream are familiar physical and psychical conditions, disputed by none and which cannot be ascribed to prepossession, dominant ideas, or diluted insanity. Therefore a profound, fearless, and searching investigation of their characteristics, causes, and operations could not fail to throw a flood of light upon many of the seeming mysteries of mental philosophy and psychology, promising a solution of some most difficult problems of life and mind, and revealing to us—as do the phenomena of dream—much of the structure and action of the Mechanism of Man.