The fourth question which suggests itself is this: Can the senses during sleep come into active operation? Touching this fourth question I reason thus: It appears as if, when one is asleep, the senses must act, for a person may move about without incurring any harm when he is in that state, as is often observed in the case of those who, like myself, walk in their sleep.... Furthermore, it has been remarked that these people are able to harness a horse and then to ride the animal safely,--acts which it is not possible to perform without the aid of the senses. On the other hand, Aristotle maintains that a man, when asleep, is not capable of using his senses. To this I reply by conceding that during sleep a man certainly does not perceive what is going on about him. Wherefore, if you answer me by saying that the mere fact of a man’s ability to walk while he is asleep furnishes conclusive evidence that he possesses his senses, I reply that movements like that of walking are not the result of an impression made upon the mind (“_impressio imaginativa_”), but the product of a different mechanism, of a nature which permits it to operate during sleep.... As to the second point to which you call attention--that, namely, with regard to the power of bridling and riding a horse while one is asleep--I make this reply: These acts are performed as a result of an impression made upon the mind through the working of the imagination, and not as a direct consequence of any images created upon the eye; for, if the sleep-walker happens to be in a strange house when the impulse to walk seizes him, he will not go to the stable. The route which he is sure to take will be one with which he is familiar, as happened in the case of the blind teacher who, unaccompanied by any person, walked habitually through the streets of Bologna. And then, besides, I am able to speak from personal experience, for in one of my sleep walks I jumped down from an elevation about four feet above the ground without awaking from my sleep.... When, in the course of one of these walks, I am exposed to cold, or when I hear somebody speaking near me, I refer these phenomena entirely to something within myself, and I return to my bed.