Foster believes that the present state of our development is not sufficient to meet the demands of continuous activity of the senses and the brain. “If it were not for fatigue,” he says, “the development of the nervous system might be carried to such a point that consciousness could be present continuously.” He finds the reason for sleep in “a temporary derangement of the nervous system.” According to Boris Sidis of Harvard, who has experimented extensively on frogs, cats, birds, dogs, children, and adults, the cells of the central nervous system, by expanding and contracting, connect themselves with, or cut themselves off from the whole nervous system, and induce “waking-states and sleeping-states.” The purely scientific man is forever aiming to reduce all the phenomena of human life to a simple formula. But no formula has yet been discovered which includes all phases of life to the satisfaction of all its students. Hence we have so many different theories of so natural and universal a function as sleep, none of them perfectly satisfactory even to their discoverers or inventors, and none affording any great help to those who want to know how to sleep. This whole neuron theory, as it is called, of dilating and contracting is really no more complete an explanation than any of the others. No perfect explanation of any natural function can be given until we can fully explain life. That has not yet been done. The most advanced biologists can say, “Here life appears,” but they cannot absolutely define life any more than they can create it out of inanimate things. We know, however, that in sleep, sight, nearly useless to man in the gloom of night, goes first to rest. Next taste goes. Then smell, still so useful to animals, deserts us. Then touch is dulled. Last of all, the hearing relaxes its guard, though with some persons it stays long awake. Noise will arouse us quicker than a touch; and last the light. As you drop off to sleep you can notice the decreased sensation in the long-serving feet which feels as if it slowly climbs to the muscles in the head and neck. CHAPTER XXIX STILL MORE THEORIES Sleep sits upon his brow; His eye is closed; he sleeps, nor dreams of harm. LONGFELLOW. We have not yet exhausted all the theories, nor shown how much too much some of them and how far too little all of them prove. The two remaining scientific theories of sleep are the psychological and the biological. The best modern exponent of the psychological theory is Marie de Manacéïne, who defines sleep as “the resting-time of consciousness.” Persons whose consciousness is but little developed, young children, and those of weak intellect, usually require a great deal of sleep, while persons whose minds are active, alert, responsive, get along with comparatively little sleep.