It has already been mentioned that the question has often been mooted whether the mind was really quite at rest during sleep, or still operative in dreams. Some authors deny its activity altogether; others admit a partial activity. The philosopher Kant went so far as to maintain that perceptions had during sleep were clearer and fuller than those of the day, because of the perfect rest of the other senses. Recollection, alone, he added, was missing, because the mind acted in sleep without the coöperation of the body. There are, however, certain facts which seem to prove that the mind does, at least, not altogether cease its activity while the body is asleep. How else could we explain the power many persons undoubtedly possess to awake at a fixed hour, and the success with which, more than once, great mental efforts have been made during profound sleep? Of the latter, Tartini's famous sonata is a striking instance. He had endeavored in vain to finish this great work; inspiration would not come, and he had abandoned the task in despair. During the night he had a dream in which he once more tried his best, but in vain; at the moment of despair, however, the Devil appeared to him and promised to finish the work in return for his soul. The composer, nothing loath, surrenders his soul and hears his magnificent work gloriously completed on the violin. He wakes up in perfect delight, goes to his desk, and at once writes down his "Devil's Sonata." Even children are known occasionally to be able to give intelligent answers while fast asleep; the questions, however, must be in accordance with the current of their thoughts, otherwise they are apt to be aroused. A case is quoted by Reil of two soldiers who used, at times, to keep up an uninterrupted conversation during a whole night, while they were to all appearances fast asleep. A lady, also, was unable to refuse answers to questions put to her at night, and had at last to lock herself in carefully whenever she went to sleep.