It is a condition of _partial unconsciousness_. In this respect it differs from the condition of coma and of trance, in which there is _entire_ unconsciousness. In the most profound sleep perfect unconsciousness never prevails. Impressions may be made upon the senses of the soundest sleeper that will waken him. The degree of oblivion caused by sleep varies immensely with various persons and with all persons at various times. Some are “light” and others “heavy” sleepers. Some are wakened by the slightest noise or the gentlest touch. Others will slumber, though rudely shaken, or while cannon are roaring. It is a remarkable fact, not yet sufficiently explained, that a whisper will often waken a sleeper by whose side a gun might be fired without disturbing him. Others will answer aloud to questions whispered to them when sleeping, and there are recorded cases of conversations being thus sustained and inconvenient revelations made by the sleeper which have astonished him on their subsequent repetition—there being in such case no after memory of the dialogue so strangely conducted. The _senses_, therefore, are but partially sealed in sleep. They are dulled, not paralysed. They convey imperfect sensations—or the sensations conveyed are imperfectly perceived—we know not which. As will be shown presently, they more or less influence mental action. They suggest dreams. But their reflex action has ceased. The nerves that convey the messages to the brain are sluggish. The nerves that convey the consequent message from the brain to the body are for the most part inactive. The aspect of the sleeper to the observer is that of unconsciousness. There are occasional motions of the limbs, but these are involuntary. He seems dead to the external world and to have ceased from active life. Nevertheless, while that form is so still and seemingly so senseless—while consciousness of a world without is suspended—in this sleep that has been called the twin brother of death—the senseless sleeper is making a world and living a life of his own within himself. That brain is not sleeping with that body. It is awake and busy—often more busy than when the body is awake. It is enacting whole dramas—living new lives—wandering away among worlds of its own creation—crowding into an hour the events of years—doing, saying, seeing, hearing, feeling, even while we gaze, a hundredfold more than the waking senses could possibly convey or the waking frame perform. Is it not marvellous when we thus think of it? Would it not be pronounced incredible—impossible—the narrator a “rogue and vagabond”—the believer a credulous fool—were it not that it is _a fact_ familiar to all of us? Is it not in itself as marvellous as any of the phenomena of other abnormal mental conditions, which are received with such incredulity and ridicule only because they are of less frequent occurrence and less familiar?