Delbœuf[16] reaches the same conclusion through a somewhat different line of argument. We give to the dream pictures the credence of reality because in sleep we have no other impressions to compare them with, because we are cut off from the outer world. But it is not perhaps because we are unable to make tests in our sleep, that we believe in the truth of our hallucinations. The dream may delude us with all these tests, it may make us believe that we may touch the rose that we see in the dream, and still we only dream. According to Delbœuf there is no valid criterion to show whether something is a dream or a conscious reality, except—and that only in practical generality—the fact of awakening. “I declare delusional everything that is experienced between the period of falling asleep and awakening, if I notice on awakening that I lie in my bed undressed” (p. 84). “I have considered the dream pictures real during sleep in consequence of the mental habit, which cannot be put to sleep, of perceiving an outer world with which I can contrast my ego.”[L] As the deviation from the outer world is taken as the stamp for the most striking characteristics of the dream, it will be worth while mentioning some ingenious observations of old Burdach[8] which will throw light on the relation of the sleeping mind to the outer world and at the same time serve to prevent us from over-estimating the above deductions. “Sleep results only under the condition,” says Burdach, “that the mind is not excited by sensory stimuli ... but it is not the lack of sensory stimuli that conditions sleep, but rather a lack of interest for the same; some sensory impressions are even necessary in so far as they serve to calm the mind; thus the miller can fall asleep only when he hears the rattling of his mill, and he who finds it necessary to burn a light at night, as a matter of precaution, cannot fall asleep in the dark” (p. 457). “The psyche isolates itself during sleep from the outer world, and withdraws from the periphery.... Nevertheless, the connection is not entirely interrupted; if one did not hear and feel even during sleep, but only after awakening, he would certainly never awake. The continuance of sensation is even more plainly shown by the fact that we are not always awakened by the mere sensory force of the impression, but by the psychic relation of the same; an indifferent word does not arouse the sleeper, but if called by name he awakens ...: hence the psyche differentiates sensations during sleep.... It is for this reason that we may be awakened by the lack of a sensory stimulus if it relates to the presentation of an important thing; thus one awakens when the light is extinguished, and the miller when the mill comes to a standstill; that is, the awakening is due to the cessation of a sensory activity, which presupposes that it has been perceived, and that it has not disturbed the mind, being indifferent or rather gratifying” (p. 460, &c.).