It is unquestionably true that dreams are sometimes evoked by sensory stimuli which almost immediately awake the dreamer. But the supposition that this fairly common fact involves an extraordinary acceleration of the rapidity with which mental images are formed is due to a failure to comprehend the conditions under which psychic activity in sleep takes place. If the sleeper were wide awake, and were suddenly startled by a mysterious voice at the window or the door, he would arrive at a theory of the sound, and even form a plan of action, with at least as much rapidity as when the stimulus occurs during sleep. The difference is that in sleep the ordinary mental associations are more or less in abeyance, and the way is therefore easily open to new associations. These new associations, when we look back at them from the standpoint of waking life, seem to us so bizarre, so far-fetched, that we think it must have required a long time to imagine them. We fail to realise that, under the conditions of dream thought, they have come about as automatically and as instantaneously as the ordinary psychic concomitants of external stimulation in waking life. It must also be remembered that in all the cases in which the rapidity of the dream process has seemed so extraordinary, it has merely been a question of visual imagery, and it is obviously quite easy to see in an instant an elaborate picture or series of pictures which would take a long time to describe.[193] At the most the dreamer has merely seen a kind of cinematographic drama which has been condensed and run together in very much the way practised by the cinematographic artist, so that although the whole story seems to be shown in constant movement, in reality the action of hours is condensed into moments. Further, it has always to be borne in mind that, asleep as well as awake, intense emotion involves a loss of the sense of time. We say in a terrible crisis that moments seemed years, and when sleeping consciousness magnifies a trivial stimulation into the occasion of a great crisis the same effect is necessarily produced. Exactly the same illusion is experienced by persons who are rescued from drowning, or other dangerous situations. It sometimes seems to them that their whole life has passed before them in vision during those brief moments. But careful investigation of some of these cases, notably by Piéron, has shown that what really happened was that a scene from childhood, perhaps of some rather similar accident, came before the drowning man's mind and was followed by five, six, perhaps even ten or twelve momentary scenes from later life. When the time during which these scenes flashed through the mind was taken into account it was found that there had by no means been any remarkable mental rapidity.