1 · With regard to sleep and waking, we must consider what they are; whether they are peculiar to soul or to body, or common to both; and if common, to what part of soul or body they appertain; further, from what cause they are attributes of animals, and whether all animals share in them both, or some partake [15] of the one only, others of the other only, or some partake of neither and some of both.
Further, we must also inquire what dreams are, and from what cause sleepers sometimes dream, and sometimes do not; or whether the truth is that sleepers always dream but do not always remember; and if this occurs, what its explanation [20] is.
Again, we must inquire whether it is possible or not to foresee the future, and if it be possible, in what manner; further, whether it extends only to things to be accomplished by the agency of men, or to those also of which the cause lies in supra-human agency, and which result from the workings of nature, or of spontaneity.
First, then, this much is clear, that waking and sleep appertain to the same part [25] of an animal, inasmuch as they are opposites, and sleep is evidently a privation of waking. For contraries, in natural as well as in all other matters, are seen always to present themselves in the same subject, and to be affections of the same: examples are—health and sickness, beauty and ugliness, strength and weakness, sight and blindness, hearing and deafness. This is also clear from the following considerations. [454a1] The criterion by which we know the waking person to be awake is identical with that by which we know the sleeper to be asleep; for we assume that one who is exercising sense-perception is awake, and that every one who is awake perceives either some external movement or else some movement within himself. If waking, then, consists in nothing else than the exercise of sense-perception, the inference is [5] clear, that that in virtue of which animals perceive, is that by which they wake, when they are awake, or sleep, when they are asleep.
But since the exercise of sense-perception does not belong to soul or body exclusively, then (since the subject of actuality is identical with that of potentiality, and what is called sense-perception, as actuality, is a movement of the soul through the body) it is clear that it is not an affection of soul exclusively, and that a soulless [10] body has not the potentiality of perception.
Now, whereas we have already elsewhere distinguished what are called the parts of the soul, and whereas the nutritive is, in all living bodies, capable of existing without the other parts, while none of the others can exist without the nutritive; it is [15] clear that sleep and waking are not affections of such living things as partake only of growth and decay, e.g. not of plants, because these have not the faculty of sense-perception, whether or not this be capable of separate existence; in potentiality, indeed, and in being it is separable.
Likewise it is clear that there is no animal which is always awake or always [20] asleep, but that both these affections belong to the same animals. For if there be an animal not endued with sense-perception, it is impossible that this should either sleep or wake; since both these are affections of the activity of the primary faculty of sense-perception. But it is equally impossible also that either of these two affections [25] should perpetually attach itself to the same animal, e.g. that some species of animal should be always asleep or always awake; for1 all organs which have a natural function must lose power when they work beyond the time for which they can work; for instance, the eyes tire of seeing, and must give it up; and so it is with the hand and every other member which has a function. Now, if sense-perception is the [30] function of something, this also, if it continues beyond the time for which it can perceive continuously, will lose its power and will do its work no longer. Accordingly, if the waking period is determined by this fact, that in it sense-perception is free; [454b1] if in the case of some contraries one of the two must be present, while in the case of others this is not necessary; if waking is the contrary of sleeping, and one of these two must be present to every animal: it must follow that the state of sleeping is necessary. Finally, if such affection is sleep, and this is a state of powerlessness [5] arising from excess of waking, and excess of waking is in its origin sometimes morbid, sometimes not, so that the powerlessness or dissolution of activity will be so or not; it is necessary that every creature which wakes must also be capable of sleeping, since it is impossible that it should always be actualizing its powers.
So, also, it is impossible for any animal to continue always sleeping. For sleep is [10] an affection of the perceptive part—a sort of bond or motionlessness—so that every creature that sleeps must have a perceptive part. Now, that which is capable of sense-perception in actuality has the faculty of sense-perception; but to actualize this faculty, in the proper and unqualified sense, is impossible while one is asleep. All sleep, therefore, must be susceptible of awakening. Accordingly, almost all [15] other animals are clearly observed to partake in sleep, whether they are aquatic, aerial, or terrestrial, since fishes of all kinds, and molluscs, as well as all others which have eyes, have been seen sleeping. Hard-eyed creatures and insects manifestly assume the posture of sleep; but the sleep of all such creatures is of brief [20] duration, so that often it might well baffle one’s observation to decide whether they sleep or not. Of testaceous animals no direct sensible evidence is as yet forthcoming to determine whether they sleep, but if the above reasoning be convincing to any one, he will be persuaded that they do.
That, therefore, all animals sleep may be gathered from these considerations. For an animal is defined as such by its possessing sense-perception; and we assert that sleep is, in a certain way, or, as it were, a motionlessness bond, imposed on [25] sense-perception, while its loosening or remission constitutes the being awake. But no plant can partake in either of these affections; for without sense-perception there is neither sleeping nor waking. But creatures which have sense-perception have likewise the feeling of pain and pleasure, while those which have these have appetite [30] as well; but plants have none of these affections. A mark of this is that the nutritive part does its own work better when the animal is asleep than when it is awake. [455a1] Nutrition and growth are then especially promoted, a fact which implies that creatures do not need sense-perception to assist these processes.
2 · We must now proceed to inquire into the cause why one sleeps and wakes, and into the particular nature of the sense-perception, or sense-perceptions, if there be several, on which these affections depend. Since, then, some animals possess all [5] the modes of sense-perception, and some not all, not, for example, sight, while all possess touch and taste, except such animals as are imperfectly developed, a class of which we have already treated in our work on the soul; and since an animal when asleep is unable to exercise, in the simple sense, any sensory faculty whatever, it follows that in the state called sleep the same affection must extend to all the senses; [10] because, if it attaches itself to one of them but not to another, then an animal while asleep may perceive with the latter; but this is impossible.
Now, since every sense has something special and also something common; special, as, e.g., seeing is to the sense of sight, hearing to the auditory sense, and so on with the other senses severally; while all are accompanied by a common power, in [15] virtue whereof a person perceives that he sees or hears (for, assuredly, it is not by sight that one sees that he sees; and it is not by taste, or sight, or both together that one discerns, and that sweet things are different from white things, but by a part common to all the organs of sense; for there is one sensory function, and the [20] controlling sensory organ is one, though differing as a faculty of perception in relation to each genus, e. g., sound or colour); and since this subsists in association chiefly with the faculty of touch (for this can exist apart from all the other organs of sense, but none of them can exist apart from it—a subject of which we have treated in our speculations concerning the soul); it is therefore evident that waking and [25] sleeping are an affection of this. This explains why they belong to all animals; for touch alone belongs to all.
For if sleeping were caused by the senses having all undergone some affection, it would be strange that these senses, for which it is neither necessary nor in a manner possible to be active simultaneously, should necessarily all go idle and [30] become motionless simultaneously. For the contrary, viz. that they should not rest simultaneously, would have been more reasonably anticipated. But, according to the explanation just given, all is quite clear regarding those also. For, when the sense organ which controls all the others, and to which all the others are tributary, [455b1] has been in some way affected, it is necessary that these others should be all affected at the same time, whereas, if one of these becomes powerless, there is no necessity for it to do so.
It is indeed evident from many considerations that sleep does not consist in the mere fact that the senses do not function or that one does not employ them, nor even [5] in the inability to exercise the sense-perceptions; for such is what happens in cases of swooning. A swoon means just such impotence of perception, and certain other cases of unconsciousness also are of this nature. Moreover, persons who have the blood-vessels in the neck compressed become insensible. But sleep supervenes when such incapacity of exercise has neither arisen in some chance organ of sense, nor from some chance cause, but when, as has been just stated, it has its seat in the [10] primary organ with which one perceives objects in general. For when this has become powerless all the other sensory organs also must lack power to perceive; but when one of them has become powerless, it is not necessary for this also to lose its power.
We must next state the cause to which sleep is due, and its quality as an affection. Now, since there are several types of cause (for we say that that for the [15] sake of which, and that whence the origin of motion comes, and the matter, and the account, are all causes), in the first place, then, as we assert that nature operates for the sake of an end, and that this end is a good; and that to every creature which is endowed by nature with the power to move, but cannot with pleasure to itself move [20] always and continuously, rest is necessary and beneficial; and since, taught by truth itself, men apply to sleep this metaphorical term, calling it a rest: we conclude that its end is the conservation of animals. But the waking state is the goal, since the exercise of sense-perception or of thought is the goal for all beings to which either of these appertains; inasmuch as these are best, and the goal is what is best. Again, [25] sleep belongs of necessity to each animal. I use the term ‘necessity’ in its conditional sense, meaning that if an animal is to exist and have its own proper nature, it must have certain endowments; and, if these are to belong to it, certain others likewise must belong to it.
The next question to be discussed is that of the kind of movement or action, [30] taking place within their bodies, from which the affection of waking or sleeping arises in animals. Now, we must assume that the causes of this affection in all other animals are identical with, or analogous to, those which operate in sanguineous animals; and that the causes operating in sanguineous animals generally are identical with those operating in man. Hence we must consider them all on this basis. Now, it has been determined already in another work that sense-perception in [456a1] animals originates in the same part in which movement originates. This is one of three determinate places, viz. that which lies midway between the head and the abdomen. This in sanguineous animals is the region of the heart; for all sanguineous [5] animals have a heart; and from this it is that both motion and the controlling sense-perception originate. Now, as regards movement, it is obvious that the origin of breathing and of the cooling process generally takes its rise there; and it is with a view to the conservation of the heat in this part that nature has provided respiration [10] and the process of being cooled by moisture. Of this per se we shall treat hereafter. In bloodless animals, and insects, and such as do not respire, the connatural spirit is seen puffed up and subsiding in the part which is in them analogous. This is clearly observable in the holoptera as wasps and bees; also in flies and such creatures. And since to move anything, or do anything, is impossible without strength, and holding [15] the breath produces strength—in creatures which inhale, the holding of that breath which comes from without, but in creatures which do not respire, of that which is connatural (which explains why winged insects, when they move, are perceived to make a humming noise, due to the friction of the connatural spirit colliding with the diaphragm of the holoptera); and since every animal moves if some sense-perception, [20] either internal or external, occurs in the primary organ of sense, accordingly if sleeping and waking are affections of this organ, the place in which and the organ in which sleep and waking originate, is evident.
Some persons move in their sleep, and perform many acts like waking acts, but not without an image or an exercise of sense-perception; for a dream is in a certain [25] way a sense-impression. But of them we have to speak later on. Why it is that persons when aroused remember their dreams, but do not remember these acts which are like waking acts, had been explained in the work on Problems.
3 · The point for consideration next in order to the preceding is:—What are [30] the processes in which the affection of waking and sleeping originates, and whence do they arise? Now, since it is when it has sense-perception that an animal must first take food and receive growth, and in all cases food in its ultimate form is, in sanguineous animals, the natural substance blood, or, in bloodless animals, that which is analogous to this; and since the veins are the place of the blood, while the [456b1] origin of these is the heart—this is clear from the dissections—it is manifest that, when the external nutriment enters the parts fitted for its reception, the exhalation enters into the veins, and there, undergoing a change, is converted into blood, and makes its way to their source. We have treated of all this when discussing the [5] subject of nutrition, but must here recapitulate what was there said, in order that we may consider the beginnings of the process, and come to know what happens to the organ of sense-perception to account for the occurrence of waking and sleep. For sleep, as has been shown, is not any given impotence of the perceptive faculty; for unconsciousness, a certain form of asphyxia, and swooning, all produce such [10] impotence. And some persons in a profound trance have still had the imaginative faculty in play. This last point, indeed, gives rise to a difficulty; for if it is possible that one who had swooned should fall asleep, the image might be a dream. Persons who have fallen into a deep trance, and have come to be regarded as dead, say many [15] things while in this condition. The same view, however, is to be taken of all these cases.
As we observed above, sleep is not any impotence of the perceptive faculty, but this affection is one which arises from the exhalation attendant upon the process of nutrition. The matter exhaled must be driven onwards to a certain point, then turn [20] back and change like a tide-race. Now, in every animal the hot naturally tends to move upwards, but when it has reached the parts above, it turns back again, and moves downwards in a mass. This explains why fits of drowsiness are especially apt to come on after meals; for the matter, both the liquid and the corporeal, which is [25] borne upwards in a mass, is then of considerable quantity. When, therefore, this comes to a stand it weighs a person down and causes him to nod, but when it has actually sunk downwards, and by its return has repulsed the hot, sleep comes on, and the animal is presently asleep. A confirmation of this appears from considering the things which induce sleep; they all, whether potable or edible, for instance [30] poppy, mandragora, wine, darnel, produce a heaviness in the head; and persons borne down and nodding all seem affected in this way, i. e. they are unable to lift up the head or the eye-lids. And it is after meals especially that sleep comes on like this, for the exhalation from the foods eaten is then copious. It also follows certain forms of fatigue; for fatigue operates as a solvent, and the dissolved matter acts, if not [457a1] cold, like food prior to digestion. Moreover, some kinds of illness have this same effect; those arising from moist and hot secretions, as happens with fever-patients and in cases of lethargy. Extreme youth also has this effect; infants, for example, [5] sleep a great deal, because of the food being all borne upwards—a mark whereof appears in the disproportionately large size of the upper parts compared with the lower during infancy, which is due to the fact that growth predominates in the direction of the former. Hence also they are subject to epileptic seizures; for sleep is like epilepsy, and, in a sense, actually is a seizure of this sort. Accordingly, the [10] beginning of this malady takes place with many during sleep, and their subsequent habitual seizures occur in sleep, not in waking hours. For when the spirit moves upwards in a volume, on its return downwards it distends the veins, and forcibly compresses the passage through which respiration is effected. This explains why [15] wines are not good for infants or for wet nurses (for it makes no difference, doubtless, whether the infants themselves, or their nurses, drink them), but such persons should drink them diluted with water and in small quantity. For wine is spirituous, and of all wines the dark more so than any other. The upper parts, in infants, are so filled with nutriment that within five months they do not even turn the neck; for in them, as in persons deeply intoxicated, there is ever a large quantity [20] of moisture ascending. It is reasonable, too, to think that this affection is the cause of the embryo’s remaining at rest in the womb at first. Also, as a general rule, persons whose veins are inconspicuous, as well as those who are dwarf-like, or have abnormally large heads, are addicted to sleep. For in the former the veins are narrow, so that it is not easy for the moisture to flow down through them; while in the case of dwarfs and those whose heads are abnormally large, the impetus of the [25] exhalation upwards is excessive. Those whose veins are large are, thanks to the easy flow through the veins, not addicted to sleep, unless, indeed, they labour under some other affection which counteracts this. Nor are the ‘atrabilious’ addicted to sleep, for in them the inward region is cooled so that the quantity of exhalation in their case is not great. For this reason they have large appetites, though spare and lean; [30] for their bodily condition is as if they derived no benefit from what they eat. The dark bile, too, being itself naturally cold, cools also the nutrient tract, and the other parts wheresoever such secretion is potentially present.
[457b1] Hence it is plain from what has been said that sleep is a sort of concentration, or natural recoil, of the hot matter inwards, due to the cause above mentioned. Hence restless movement is a marked feature in the case of a person when drowsy. But where it begins to fail, he grows cool, and owing to this cooling process his eye-lids droop. Accordingly the upper and outward parts are cool, but the inward [5] and lower, i. e. the parts at the feet and in the interior of the body, are hot.
Yet one might found a difficulty on the facts that sleep is most oppressive in its onset after meals, and that wine, and other such things, though they possess heating properties, are productive of sleep, for it is not probable that sleep should be a process of cooling while the things that cause sleeping are themselves hot. Is the [10] explanation of this, then, to be found in the fact that, as the stomach when empty is hot, while replenishment cools it by the movement it occasions, so the passages and tracts in the head are cooled as the exhalation ascends thither? Or, as those who have hot water poured on them feel a sudden shiver of cold, just so in the case before [15] us, may it be that, when the hot substance ascends, the cold rallying to meet it cools them, deprives their native heat of all its power, and compels it to retire? Moreover, when much food is taken, which the hot substance carries upwards, this latter, like a fire when fresh logs are laid upon it, is itself cooled, until the food has been digested.
For, as has been observed elsewhere, sleep comes on when the corporeal [20] element is conveyed upwards by the hot, along the veins, to the head. But when that which has been thus carried up can no longer ascend, but is too great in quantity it forces the hot back again and flows downwards. Hence it is that men sink down when the heat which tends to keep them erect (man alone, among animals, being naturally erect) is withdrawn; and this, when it befalls them, causes unconsciousness, [25] and afterwards imagination.
Or are the solutions thus proposed possible accounts of the refrigeration which takes place, while, as a matter of fact, the region of the brain is, as stated elsewhere, the main determinant of the matter? For the brain, or in creatures without a brain that which corresponds to it, is of all parts of the body the coolest. Therefore, as [30] moisture turned into vapour by the sun’s heat is, when it has ascended to the upper regions, cooled by the coldness of the latter, and becoming condensed, is carried downwards, and turned into water once more; just so the waste exhalation, when [458a1] carried up by the heat to the region of the brain, is condensed into phlegm (which explains why catarrhs are seen to proceed from the head); while that exhalation which is nutrient and not unwholesome, becoming condensed, descends and cools [5] the hot. The tenuity or narrowness of the veins about the brain itself contributes to its being kept cool, and to its not readily admitting the exhalation. This, then, is a sufficient explanation of the cooling which takes place, despite the fact that the exhalation is exceedingly hot.
A person awakes from sleep when digestion is completed: when the heat, which [10] had been previously forced together in large quantity within a small compass from out the surrounding part, has once more prevailed, and when a separation has been effected between the more corporeal and the purer blood. The finest and purest blood is that contained in the head, while the thickest and most turbid is that in the lower parts. The source of all the blood is, as has been stated both here and [15] elsewhere, the heart. Now of the chambers in the heart the central communicates with each of the two others. Each of the latter again acts as receiver from each of the two veins, the one called the ‘great’ and the ‘aorta’. It is in the central chamber [20] that the separation takes place. To go into these matters in detail would, however, be more properly the business of a different treatise from the present. Owing to the fact that the blood formed after the assimilation of food is especially in need of separation, sleep occurs until the purer part of this blood has been separated off into the upper parts of the body, and the more turbid into the lower parts. When this has taken place animals awake from sleep, being released from the heaviness [25] consequent on taking food.
We have now stated the cause of sleeping, viz., that it consists in the recoil by the corporeal element, borne upwards by the connatural heat, in a mass upon the primary sense-organ; we have also stated what sleep is, having shown that it is a seizure of the primary sense-organ, rendering it unable to actualize its powers; [30] arising of necessity (for it is impossible for an animal to exist if the conditions which render it an animal be not fulfilled), i.e., for the sake of its conservation; since remission of movement tends to the conservation of animals.