Sometimes these, sleep and waking, are injurious,—at other times beneficial. For sleep and also waking bring forth diseases, intensify them; both equally drive them away, soothe sorrows and likewise intensify them; by one and the other alike, morbid causes are often destroyed more effectually than by any other remedy; indeed, in conjunction with the benefit of these (sleep and waking), concoctions, food, purgatives, and finally all the functions of the different parts of the body may be exercised to the best advantage; nor is it possible, indeed, for a creature to live, or to maintain his life, without sleep and waking. There is no action of the body or mind which has greater values to the body, nothing which supplies more reliable signs for discerning bodily ills, and showing how to be rid of them. Of which things, indeed, the investigation and knowledge is most useful, and not without pleasure to those who delight in the understanding of things; that is what Aristotle, prince of philosophers, notices, when he writes his whole book concerning sleep and waking,—and often elsewhere at random in his writings. Not the less does Hippocrates notice them in his citations, for he wrote most sayings on the subject, so many that I omit them; and there are many in other books, of which a definite impression does not remain in my memory. But as I have said, when all, or certainly most writers on this subject may be perplexed with regard to these things, and involved in many difficulties, no one ought to condemn me, if, after them, and many men, I presume to write on this subject. CHAPTER I BY WHAT CAUSES AND MEANS SLEEP AND WAKING ARE PRODUCED, ACCORDING TO THE OPINION OF ARISTOTLE To refute the opinion of the philosophers concerning the causes of sleep and waking, I think superfluous; because, with Aristotle’s views surviving now many centuries, no authority among these other writers may be greater than his; and because the ignorant premises of the others makes all discussion of them become inane. Because of this I think I should be excused for introducing the opinion of Aristotle among all the philosophers, that is, for choosing it from among them, for if we show it to be equally probable with the others which we presume to refute, it will be because, unlike them, it extinguishes them by its own plausibility. Aristotle thinks sleep to be produced by a vapor generated by the heat energy in food, the fumes of which, rising to the brain, are there converted into moisture by the coldness of the brain itself, because, as he says, cold is felt at the contact just as rain is known to be made from vapor in contact with cold air; which moisture or humor, by force of gravity, is pushed downward, descends through the veins, drives the heat from the heart, and makes that cold also; whence, the cold spreading about, sleep, he says, generally arises.