The physician is to be called in not only when we are dealing with conditions of sleep and dream which are in the least diseased, i. e., abnormally intense sleepiness, sleep-walking, hallucinatory dreams, etc., but also when the physiological side of sleep and dream are in question, e. g., the need of sleep, the effect of insomnia, of normal sleepiness, etc. The criminalist must study also these things in order to know the kind of situation he is facing and when he is to call in the physician for assistance. Ignorance of the matter means spoiling a case by unskilful interrogation and neglect of the most important things. At the very least, it makes the work essentially more difficult.
But in many cases the criminalist must act alone since in those cases there is neither disease nor a physiological condition by way of explanation but merely a simple fact of the daily life which any educated layman must deal with for himself. Suppose, e. g., we are studying the influence of a dream upon our emotions. It has been shown that frequently one may spend a whole day under the influence of a dream, that one's attitude is happy and merry as if something pleasant had been learned, or one is cross, afraid, excited, as if something unhappy had happened. The reason and source of these attitudes is frequently a pleasant or unpleasant dream, and sometimes this may be at work subconsciously and unremembered. We have already shown that so-called errors of memory are to a large extent attributable to dreams.
This effect of the dream may be of significance in women, excitable men, and especially in children. There are children who consider their dreams as real experiences, and women who are unable to distinguish between dreams and real experience, while the senile and aged can not distinguish dreams and memories because their memories and the power to distinguish have become weakened.
I know of an eight-year-old child who after dinner had gone looking for chestnuts with a man. In the evening it came home happy but woke up in tears and confessed that the man in question had
 Cf. S. Freud: Traumdeutung. Leipzig 1900 (for the complete bibliography). B. Sidis: An Experimental Study of Sleep: Journal of Abnormal Psychology.
 Maudsley. Physiology and Pathology of the Mind.
 Cf. Altmann in H. Gross's Archiv. I, 261.
raped it. Another case concerns a great burglary which had caused its victims considerable excitement. The second day after the event the ten or twelve-year-old daughter of the victim asserted with certainty that she had recognized the son of a neighbor among the thieves. In both cases there were serious legal steps taken against the suspects, and in both cases the children finally admitted, after much thinking, that they had possibly dreamed the whole matter of their complaints.