On Sleep and Waking

​Since there are several ways of cause (for we say that the end, that from which the source of motion comes, the matter, and the definition are causes), first of all, since we say that nature acts for the sake of something, and this is some good, and that for everything whose nature is to move, but which cannot move always and continuously with pleasure, rest is necessary and beneficial, they apply this metaphor to sleep on account of the truth itself, as being a rest; so that it exists for the sake of the preservation of animals. But waking is the end; for perceiving and thinking are the end for all those in which either of them belongs; for these are best, and the end is the best. So that it is necessary for sleep to belong to each of the animals. And I speak of necessity on hypothesis, that if there is to be an animal having its own nature, it must of necessity have certain things belonging to it, and, these belonging to it, others must belong. Furthermore, what kind of motion and action taking place in their bodies causes waking and sleeping to happen to animals, must be said after this. Now, for the other animals, just as for the red-blooded ones, we must assume that the causes of the affection are either the same or analogous, and for the red-blooded ones the same as for humans; so that from these we must investigate all things. That the source of perception arises from the same part in animals from which the source of motion also comes, has been determined before in other works. And this is the middle of three determined regions, that is, of the head and the lower belly. For red-blooded animals, then, this is the region around the heart; for all red-blooded animals have a heart, and the source of motion and of the primary perception is from there. As for motion, it is clear that both the source of breath and generally that of cooling is there, and nature has provided both breathing and cooling by moisture for the preservation of the heat in this part. But about it, it will be said later by itself. For bloodless animals and insects and those which do not receive breath, the connate breath is seen to expand and contract in the analogous part. And this is clear in the case of the insects with undivided wings, such as wasps and bees, and in flies and whatever other such creatures. Since, however, it is impossible to move something or to act without strength, and strength is produced by the holding of breath, by the external breath for those that draw it in, and by the connate breath for those that do not breathe. Wherefore also the winged creatures are seen to buzz when they move, by the friction of the breath striking against the waist-boundary of the insects with undivided wings. And everything moves when some perception, either internal or external, occurs in the primary sense-organ. If, then, sleep and waking are affections of this part, in what region and in what primary part sleep and waking occur is clear. And some people move while sleeping and do many things characteristic of waking, yet not without an image and some perception; for a dream is in a way a perception. But about them it must be said later. And why indeed they remember dreams when they wake up, but do not remember their waking-like actions, has been said in the Problems.