For a long time it was believed that living creatures devoid of consciousness would not sleep at all, but recent experiments have apparently weakened that conclusion. Dogs, pigeons, and other animals deprived of brains in the interest of scientific discovery, appear to sleep, that is, they have periods of inactivity, just as those with brains and consciousness. Belmondo, after repeated experiments, drew the conclusion that sleep is not a purely cerebral function, as some believe, but that the whole organism sleeps; and the brain sleeps only because the organs of sense sleep. This, however, is doubtful. And this is in sum and substance the biological theory of sleep, that the whole organism sleeps, but even here there are exceptions. It is true that the heart beats less rapidly; that we breathe less frequently; that the brain cells cease their functioning because the neuroglia contracts and shuts off or lessens their joint activity; the motor consciousness rests; the nerves of sensation refuse to be stimulated, we sleep. Yet we know that the spinal cord never sleeps, that certain functions of the body continue uninterruptedly in the sleeping-state as in the waking-state, and, after all these years of theorizing and experimenting, we do not know definitely what sleep is. We know the mechanism of sleep, its manifestations and its effects; we know that continued sleeplessness means madness and death; that sleep is essential to the physical and mental well-being of the human organism, but we do not know what sleep is any more than we know what life is. There is a limit to what material science can know. Heubel, who is one of the recent supporters of the psychological theory of sleep, says that “Mental activity depends on the incoming peripheral sensory stimulations; when such peripheral sensory stimulations are absent, mental activity is in abeyance and sleep results.” This is, in effect, to say that, when things about us no longer give us any sensation, when they do not attract or hold our attention, we fall asleep. But we all know of exceptions to this rule. We have seen others fall into “a brown study,” and have probably done so ourselves, and become perfectly lost to all about them; absorbed in their own reflections, they neither hear nor see the things happening around them. For the time being “peripheral sensory stimulations” are absent, and yet mental activity continues and sleep does not result.