This is the subject of a lecture delivered before the Institut Psychologique. The part played in dreams by visual sensations of internal and external origin is all-important, but auditory sensations as well often determine the character of certain dreams. And so with sensations of touch. The author mentions interesting observations illustrating these points. One must remember that in what we call natural sleep our senses continue to work. Although they work with less precision they receive numerous impressions, which, while they would attract no notice during the waking period, may be vivid during sleep. The colours, the changing forms, which appear when our eyes are closed, constitute the material of our dreams; they do not produce them, because they are vague and ill-defined. Memory forms our dreams. Recollections of objects perceived more or less clearly, more or less difficult to recall during the waking state; these give shape to our dreams, although we cannot always recognise this. A conjunction of the two factors, memory and sensation, constitutes the dream. The author also examines the question of the psychological characteristic of sleep, the real or essential difference between perceiving and dreaming. It is not abolition of reasoning. To sleep is to be disinterested. In dreaming the same faculties are exercised as during the waking state, but they are in a state of relaxation, not in a state of tension. We hear a dog barking during sleep; we dream in consequence of an assembly murmuring, shouting, etc.; no effort is required. To associate the noise with the barking of a dog requires a positive effort. This force the dreamer lacks, and herein he differs from the subject awake. Other differences might be deduced from this essential difference. The author mentions especially three points: the incoherence of dreams, the abolition of the sense of duration which dreams often appear to manifest, and the order in which recollections appear before the dreamer, to fit in with the sensations actually present.