1093

This view is not traversed by the fact that there are marginal cases in which the dream - as happens with anxiety-dreams - can no longer perform its function of preventing an interruption of sleep, but assumes instead the other function of promptly bringing sleep to an end. In doing so it is merely behaving like a conscientious night-watchman, who first carries out his duty by suppressing disturbances so that the townsmen may not be woken up, but afterwards continues to do his duty by himself waking the townsmen up, if the causes of the disturbance seem to him serious and of a kind that he cannot cope with alone.

The function of the dream as a guardian of sleep becomes particularly evident when an external stimulus impinges upon the senses of a sleeper. It is generally recognized that sensory stimuli arising during sleep influence the content of dreams; this can be proved experimentally and is among the few certain (but, incidentally, greatly overvalued) findings of medical investigation into dreams. But this finding involves a puzzle which has hitherto proved insoluble. For the sensory stimulus which the experimenter causes to impinge upon the sleeper is not correctly recognized in the dream; it is subjected to one of an indefinite number of possible interpretations, the choice being apparently left to an arbitrary psychical determination. But there is, of course, no such thing as arbitrary determination in the mind. There are several ways in which a sleeper may react to an external sensory stimulus. He may wake up or he may succeed in continuing his sleep in spite of it. In the latter case he may make use of a dream in order to get rid of the external stimulus, and here again there is more than one method open to him. For instance, he may get rid of the stimulus by dreaming that he is in a situation which is absolutely incompatible with the stimulus. Such was the line taken by a sleeper who was subject to disturbance by a painful abscess on the perineum. He dreamt that he was riding on a horse, making use of the poultice that was intended to mitigate his pain as a saddle, and in this way he avoided being disturbed. Or, as happens more frequently, the external stimulus is given an interpretation which brings it into the context of a repressed wish which is at the moment awaiting fulfilment; in this way the external stimulus is robbed of its reality and is treated as though it were a portion of the psychical material. Thus someone dreamt that he had written a comedy with a particular plot; it was produced in a theatre, the first act was over, and there were thunders of applause; the clapping was terrific. . . . The dreamer must have succeeded in prolonging his sleep till after the interference had ceased; for when he woke up he no longer heard the noise, but rightly concluded that someone must have been beating a carpet or mattress. Every dream which occurs immediately before the sleeper is woken by a loud noise has made an attempt at explaining away the arousing stimulus by providing another explanation of it and has thus sought to prolong sleep, even if only for a moment.