Aristotle · a new plain-English translation from the original language
1| ON SLEEP AND WAKING. We must investigate sleep and waking: what they actually are, and whether they are proper to the soul or to the body or common to both, and if common, to which part of the soul or of the body; and for what cause they belong to animals; and whether all animals share in both of these, or some in sleep only and others in waking only, or some in neither and others in both. Besides this, what is a dream, and for what cause the sleeping sometimes dream and sometimes do not; or does it happen that those who sleep always dream, but do not remember it; and if this happens, for what cause it happens; and whether it is possible to foresee future events, or not possible; and in what way, if it is possible; and whether this applies only to things that are going to be done by a human being, or also to things whose cause lies with the divine and that come about by nature or by chance. First, then, this much at least is clear: that waking and sleep belong to the same thing in the animal, for they are opposites, and sleep appears to be a certain privation of waking. For opposites are always seen to occur in the same recipient and to be affections of the same thing, both elsewhere and in the domain of natural things — I mean, for example, health and disease, beauty and ugliness, strength and weakness, sight and blindness, hearing and deafness.
2| Further, this is also clear from the following. That by which we recognize the one who is awake is the same thing by which we recognize the one who is asleep. For we think that the one who perceives is awake, and that every waking thing perceives either something external or some motions within itself. If, then, being awake is nothing other than perceiving, it is clear that it is by the very thing by which it perceives that a waking thing is awake and a sleeping thing sleeps. Now since perceiving is proper neither to the soul alone nor to the body alone (for that of which there is a capacity, of that there is also an actuality; and what is called perception, as an actuality, is a certain motion of the soul through the body), it is clear that this affection is proper neither to the soul alone, nor is a soulless body capable of perceiving. Since it has been determined earlier, in other works, about the things called parts of the soul, and since the nutritive part is separate from the others in things that have life in their bodies, while none of the others occurs without this one, it is clear that whatever living things partake only of growth and decay have neither sleep nor waking — plants, for example. For they do not have the perceptive part, whether it is separable or not separable; for it is separable in capacity and in being.
3| Likewise it is also clear that there is nothing that is always awake or always asleep, but both these affections belong to the same animals. For it is not possible that anything which is an animal having perception should be incapable both of sleeping and of waking; for both these affections concern the perception belonging to the primary perceptive faculty. Nor is it possible for either of these to belong always to the same thing — for example, that some kind of animal should always be asleep or always awake. Further, whatever things have some function according to nature, when that function exceeds the time for which it is capable of doing something, must become incapable of it — for example, the eyes, which see and also cease from doing this, and likewise the hand and everything else that has some function. If, then, perceiving is the function of something, and this too, if it exceeds the time for which it was capable of perceiving continuously, will become incapable and will no longer do this. If, then, being awake is defined by the perceptive faculty being unbound, and of two opposites one must necessarily be present while the other need not, and being awake is the opposite of sleeping, and one or the other must necessarily belong to everything, it would be necessary to sleep. If, then, this affection is sleep, and this is an incapacity due to an excess of waking, and the excess of waking sometimes comes about through disease and sometimes without disease, so that the incapacity too, and the dissolution, will occur in the same way, it is necessary that everything that is awake be capable of sleeping;
4| for it is impossible to be always in actuality. Likewise nothing is capable of always being asleep either. For sleep is a certain affection of the perceptive part, a kind of binding and immobilization, so that everything that sleeps must necessarily have the perceptive part. And the perceptive is what is capable of perceiving in actuality; but it is impossible to be in actuality by perception, in the strict and unqualified sense, and to be asleep at the same time; hence it is necessary that every sleep be one from which one can be woken. Now as for the other animals, it is virtually clear that they all share in sleep, both swimming and flying and walking creatures; for all the kinds of fish and of soft-bodied creatures have been observed sleeping, and all the rest that have eyes; for hard-eyed creatures too are evidently seen sleeping, and insects as well. All such creatures, however, sleep briefly, so that it might often escape notice whether they partake of sleep or not. As for the shelled creatures, it has not yet become clear from perception whether they sleep; but if the argument just given seems persuasive to anyone, he will be convinced by it. That all animals, then, share in sleep is clear from these considerations; for the animal is defined by having perception, and of perception we say that a certain immobility and, as it were, a binding is sleep, while its release and relaxation is waking.
5| None of the plants can share in either of these affections. For without perception there is neither sleep nor waking; and whatever has perception has also pain and pleasure; and whatever has these has also appetite. But none of these belong to plants. A sign of this is that the nutritive part performs its own function better during sleep than during waking; for at that time nourishment and growth occur more, since these do not need perception in addition.
6| We must investigate why an animal sleeps and wakes, and through what sort of perception, or what sorts, if through several. Now since some animals have all the perceptions while others do not — sight, for instance — but all have touch and taste, except perhaps some incomplete animal (this has been discussed in the works on the soul), and it is impossible for a sleeping animal to perceive, without qualification, by any perception whatsoever, it is clear that the same affection must necessarily belong to all in what is called sleep; for if an animal perceived by one perception while sleeping but not by another, it would in that respect be perceiving while asleep, and this is impossible. Now since in each perception there is something proper to it and something common, proper being, for example, seeing for sight and hearing for hearing, and similarly for the others in the same way; and there is also a certain common capacity that follows upon all of them, by which one is also aware that one sees and hears and perceives — for it is surely not by sight itself that one sees that one sees. And indeed one also judges, and is able to judge, that sweet things are different from white things, not by taste, nor by sight, nor by both together, but by some common part of all the organs of perception.
7| For perception is one thing, and the controlling organ of perception is one; but what it is for a perception of each kind — for example, of sound and of color — to occur is different. This common capacity belongs together with the organ of touch above all; for touch is separable from the other organs of perception, while the others are inseparable from it. This has been discussed in the treatises on the soul. It is clear, then, that waking and sleep are an affection of this organ. And this is why they belong to all animals; for touch alone belongs to all. For if being asleep came about by all the perceptions being affected in some way, it would be strange that those perceptions which neither must nor can in any way be in actuality at the same time should necessarily be idle and immobilized together at the same time; for it would be more reasonable that the opposite should happen to them — that they should not be at rest all together. But as we are now saying, the matter stands reasonably even with regard to these; for when the organ that controls all the others, and toward which the rest converge, is affected in some way, it is necessary that all the rest be affected along with it, whereas when one of those others becomes incapable, it is not necessary for this controlling organ to become incapable. It is clear from many considerations that sleep does not consist in the perceptions being idle and not being used, nor in being unable to perceive.
8| For this is what happens in fainting fits too, since a fainting fit is an incapacity of the senses. Certain kinds of loss of consciousness also come about in this way. Again, those whose neck veins are compressed become without perception. But sleep occurs when the incapacity for the exercise of sense arises not in some chance sense organ nor from some chance cause, but, as has just now been said, in the first organ by which the animal perceives all things: for when this becomes incapacitated, it is necessary that all the sense organs be incapacitated for perceiving as well, whereas when one of those others is affected, this first organ need not be. We must now state the cause on account of which sleep occurs, and what sort of affection it is. Since there are several senses of cause (for we say that the cause is that for the sake of which, and that from which the origin of motion comes, and the matter, and the account), then first, since we say that nature does things for the sake of something, and this something is a good, and that for everything whose nature it is to move but which cannot move always and continuously, rest with pleasure is both necessary and beneficial — people apply this metaphor to sleep, calling it a rest, precisely because it is true that sleep is a kind of rest — sleep therefore exists for the sake of the preservation of animals. Waking, on the other hand, is an end.
9| For perceiving and thinking are an end for all things to which either of them belongs, since these are the best things, and the end is the best thing. So it is necessary that sleep belong to each animal. I mean the necessity hypothetically: that if there is to be an animal having its own nature, certain things must necessarily belong to it, and if these belong, certain other things must belong as well. Further, we must next state what sort of motion and activity occurring in the bodies produces waking and sleeping in animals. For the other animals, then, just as for the blooded ones, we must suppose the causes of the affection to be either the same or analogous, and for the blooded ones the same as for human beings; so we must study everything on this basis. That the origin of perception occurs in animals from the same part from which the origin of motion also comes has been determined earlier, elsewhere. This is, of three distinguished regions, the middle one, between the head and the lower belly. For the blooded animals this is the region around the heart; for all blooded animals have a heart, and the origin both of motion and of the authoritative perception is from there.
10| Now as regards motion, it is clear that the origin of the breath, and in general of the cooling, is there as well, and nature has provided both breathing and cooling by moisture for the preservation of the heat in this part. This will be discussed later on its own account. In the bloodless animals, and in the insects and those that do not take in breath, the connate breath is seen, in the analogous part, to swell and to subside. This is clear in the case of the wing-cased insects, such as wasps and bees, and in flies and all such creatures. And since it is impossible to move or act on anything without force, and the retention of breath produces force — for those that draw it in, force from outside, and for those that do not breathe, the connate breath — this is why winged creatures are seen to buzz when they move, from the friction of the breath striking against the diaphragm-like membrane of the wing-cased insects. Everything that moves does so as some perception occurs, either its own or another's, in the first sense organ. If sleep and waking are affections of this part, it is clear in what place and in what first part sleep and waking occur.
11| Some creatures move while asleep and do many things characteristic of waking, yet not without some image and some perception; for the dream is in a way a perception. This must be discussed later. As for why people remember their dreams once they have woken but do not remember their waking activities, this has been discussed in the Problems. Next after what has been said, it is proper to go on to examine what things occur, and from where the origin of the affection arises, both of waking and of sleeping. It is clear that, since it is necessary for the animal, once it has perception, then for the first time to take in nourishment and grow, and nourishment for all animals is, in its final form, for the blooded ones the nature of blood, and for the bloodless ones the analogous substance, and the place of the blood is the veins, and the origin of these is the heart — and what has been said is clear from dissections. Now when the nourishment from outside enters into the receptive places, the exhalation rises into the veins, and there, changing, it is turned into blood and travels to the origin. This has been discussed in the works on nourishment; but now it must be taken up again for this purpose, so that we may examine the origins of motion, and what happens to the perceptive part when waking and sleep occur. For sleep is not just any incapacity of the perceptive part whatsoever, as has been said; for loss of consciousness, and a kind of choking, and fainting also produce such an incapacity. And it has already happened to some people, through mere imagination, that they fainted violently. Now this presents a certain difficulty:
12| for if it is possible for one who has fainted to fall asleep, then it would be possible for the image too to be a dream. There are many things that those who have fainted very severely and seemed to be dead report; about all of these the same account must be supposed to hold. But in fact, as we said, sleep is not every incapacity of the perceptive part, but this affection arises from the exhalation connected with nourishment; for what is exhaled must necessarily be pushed up to a certain point, and then turn back and change direction, like a strait with reversing currents. The heat of each animal is naturally borne upward; but when it comes to be in the upper regions, it turns back all at once again and is carried down. This is why sleep occurs most of all from food; for then a great deal of moisture and bodily matter is carried up all at once. While it is rising it weighs the creature down and makes it nod off; but when it inclines downward and, having turned back, pushes away the heat, then sleep occurs and the animal sleeps. A sign of this is also found in things that induce sleep; for all of them produce heaviness in the head, both drinks and foods — poppy, mandrake, wine, darnel. And those who are sinking down and nodding off appear to undergo this very thing, and are unable to lift their head or their eyelids. And sleep of this kind occurs especially after meals;
13| for the exhalation from food is considerable. It also occurs from certain kinds of fatigue; for fatigue causes a kind of melting, and the melted matter becomes like undigested nourishment, unless it is cold. Certain diseases also produce this same effect, namely those that arise from a moist and hot residue, as happens to people with fevers and in cases of lethargy. Further, there is the earliest stage of life; for small children sleep a great deal because all their nourishment is carried upward. A sign of this is that the upper parts exceed the lower in size during this earliest stage of life, because growth occurs in that direction. For this same reason children are also prone to epileptic fits; for sleep is similar to epilepsy, and sleep is in a way a kind of epilepsy. This is why in many people the onset of this affection occurs while they are asleep, and they are seized by it while asleep but not while awake; for when a great deal of breath is carried upward, and comes back down again, it swells the veins, and compresses the passage through which breathing occurs. This is why wine does not agree with small children, nor with their nurses (for it perhaps makes no difference whether the children themselves drink it or the nurses do), but they should drink watered-down wine, and little of it;
14| For wine is full of vapor, and dark wine more so than the rest. And the upper parts are so full of nourishment in infants that up to five months they cannot even turn their necks; for just as in those who are very drunk, a great deal of moisture is carried upward. It is reasonable that this affection is also the cause of embryos lying still in the womb at first. And in general, those whose veins are indistinct, and dwarfish people, and people with large heads, are fond of sleep; for in the former the veins are narrow, so that it is not easy for the fluid to flow down through them, while in dwarfish and large-headed people the upward impulse and the rising vapor are considerable. But people with prominent veins are not prone to sleep, on account of the free flow through their veins, unless they have some other affection working against this. Nor are melancholic people prone to sleep; for the inner region is chilled in them, so that no abundance of vapor arises in them. This is also why, though they are hardy eaters, they remain lean; for their bodies are disposed as if they had derived no benefit from their food. Black bile, being cold by nature, also makes the nutritive region cold, and the other parts as well, wherever this residue is present potentially. So it is clear from what has been said that sleep is a kind of concentration of the hot inward, a natural counter-reaction, for the cause stated.
15| For this reason there is much motion in the person sleeping. And where the heat departs from, that place is chilled, and it is on account of this chilling that the eyelids fall shut. And the upper parts and the outer parts are chilled, while the inner and lower parts are hot, as around the feet and the internal parts. Yet one might raise a difficulty here, that sleep becomes strongest after food, and that wine and other things possessing such heat are conducive to sleep. But it is not reasonable that sleep should be a chilling while the causes of sleeping are hot. Is it, then, that this happens in the way that the stomach, when empty, is hot, but filling it chills it because of the motion involved, and likewise the passages and places in the head are chilled as the vapor is carried upward? Or is it as with those who have hot liquid suddenly poured over them, who get a shudder — so there too, as the heat rises, the cold that gathers chills it, and makes the natural heat grow powerless and withdraw? Further, when much nourishment falls in, which the heat draws up — just as fire is chilled when logs are piled on it — the body is chilled until the food is concocted. For sleep occurs, as has been said, when the corporeal element is carried up by the heat through the veins toward the head.
16| But when the heat can no longer manage this, and what has been carried up exceeds it in quantity, it is pushed back again and flows downward. This is why, as the heat that was carrying it upward is withdrawn, people fall down (for humans alone among animals stand upright), and its falling produces first a loss of consciousness, and afterward imagination. Now the explanations just given are capable of accounting for how the chilling occurs; nevertheless, the region around the brain is the governing factor, as has been said elsewhere. Of all the parts in the body the brain is the coldest — or, in animals lacking a brain, the part analogous to it. Just as moisture that is vaporized by the heat of the sun, when it comes to the upper region, is chilled on account of that region's coldness and, condensing, is carried back down again as water, so too in the rising of heat toward the brain, the residual vapor collects into phlegm (which is why catarrhs are evidently produced from the head), while the nourishing vapor that is not diseased is carried back down as it condenses and chills the heat. And toward this chilling, and toward not readily admitting the vapor, the fineness and narrowness of the veins around the brain also contribute.
17| This, then, is the cause of the chilling, even though the vapor exceeds the surrounding region in heat. Waking occurs when the heat that has been forced together is concocted and, being much in a small space, gains mastery from what surrounds it, and the blood that is more corporeal is separated out from what is purest. The blood in the head is finest and purest, while that in the lower parts is thickest and most turbid. The source of all blood, as has been said both here and elsewhere, is the heart. Of the chambers in the heart, the middle one is common to the two on either side; and each of those two receives from each of the two vessels from each of the two vessels, the one called the great vessel and the aorta; and it is in the middle one that the separating-out occurs. But to determine matters about this belongs more properly to other discussions. And because the blood becomes less well separated after the intake of food, sleep occurs, until the blood is separated, the purer part going to the upper region and the more turbid part to the lower; and when this happens, people wake, released from the weight arising from their food. What the cause of sleeping is, then, has been stated: it is the counter-reaction, occurring all at once at the primary organ of perception, produced by the corporeal element that is carried up by the connate heat.
18| And what sleep is: it is a seizing of the primary organ of perception such that it is unable to be active, occurring of necessity (for it is not possible for an animal to exist unless the conditions that produce the animal are fulfilled), but for the sake of preservation; for rest preserves it.