To remark that one is not a constant dreamer is not to assert that dreaming is rare, but merely that one's recollection of it is rare. Though we may only catch a glimpse of our latest vision of the night as we leave the house of sleep, it may well be that there were many earlier adventures of the night which are beyond the reach of waking consciousness. Sometimes, it is curious to note, we become vaguely conscious, during the day, for the first time, of a dream we have had during the night. Many psychologists, as well as metaphysicians--fearful to admit that the activity of the soul could ever cease--believe that we dream during the whole period of sleep; this has of recent years been the opinion of Vaschide, Foucault, Näcke, and Sir Arthur Mitchell, as it formerly was of Sir Benjamin Brodie, Sir Henry Holland, and Schaaffhausen. In earlier days Hippocrates, Descartes, Leibnitz, and Cabanis seem to have been of the same opinion. On the other hand, Locke, Macnish, and Carpenter held that deep sleep is dreamless; this is also the opinion of Wundt, Beaunis, Strümpell, Weygandt, Hammond, and Jastrow. Moreover, there are some people, like Lessing, who, so far as they know, never dream at all. My own personal experience scarcely inclines me to accept without qualification the belief that we are always dreaming during sleep. I find that my remembered dreams tend to be correlated with some slight mental or physical disturbance, and therefore it seems to me probable that, if dreams are continuous during sleep, they must, during completely undisturbed sleep, be of an extremely faint and shadowy character. To return to a metaphor I have before used, we may say that sleeping consciousness in its descent from the surface of the waking life may fall to a point at which its specific gravity being practically the same as that of its environment, a state approaching complete repose is attained.[8] It cannot of course be said that the failure to remember dreams is any argument against their occurrence. It is well known that when the psychic activity of sleep assumes a definitely motor shape, as in talking in sleep and in somnambulism, it is very rare for any recollection to remain on awakening, though we cannot doubt that psychic activity has been present. In the same way the dream that we remember when awakened from sound sleep by another person is by no means always due to that awakening. This is shown by the fact that if we were turning round or making other movements just before being thus awakened, the dream we remember--in one such case a dream of making one's way with difficulty between a sofa and a chair--may have no relation to the circumstance of the awakening, but clearly be suggested by the movements made during sleep, though these movements themselves remain unknown to waking consciousness. The movements of dogs during sound sleep--the rhythmical lifting of the paws, the wagging of the tail--point in the same direction.[9]