When we are sleeping naturally, it is not necessary to believe, as has often been supposed, that our senses are locked to external sensations. Our senses continue to be active. They act, it is true, with less precision, but in compensation they embrace a host of "subjective" impressions which pass unperceived when we are awake—for then we live in a condition of perceptions common to all men—and which reappear in sleep, when we live only for ourselves. Thus our faculty of sense perception, far from being narrowed during sleep at all points, is on the contrary extended, at least in many directions, in its field of operations. It is true that it often loses in situation, in tension, what it gains in extension. It brings to us only confused impressions. These impressions are the materials of our dreams. But they are only the materials, they do not suffice to produce them.