De Somno et Vigilia

Part 1

Sleep is a privation of waking and, inasmuch as they are opposites, sleep and waking must appertain to the same part of an animal. Moreover, the criterion of sense perception by which a waking person is judged to be awake is identical to that by which a sleeping person is judged to be asleep. If waking consists of nothing other than the exercise of sense perception, then the organ by which animals sleep or wake is the same as that by which they perceive. Sense perception is a movement of the soul through the body; as such, it is neither an exclusive property of the soul nor an exclusive property of the body.

Living things such as plants that partake of growth but that do not have the faculty of sense perception do not sleep or wake. Of those living things that do wake or sleep, there is none that is either always asleep or always awake. Organs lose power when they are over-worked, and so it is also with the organ of sense perception. It is impossible for any animal to perpetually actualise its powers, for which reason every animal that wakes must also sleep. Conversely, the faculty of sense perception exists to be exercised, and every animal that sleeps must also wake. Almost all animals have been observed to partake in sleep, whether they are aquatic, aerial, or terrestrial. Not so testaceous animals (animals with a firm, calcareous shell such as oysters and clams), although our reasoning leads us to suppose that they must. By definition, an animal is any creature with sense perception. Creatures with sense perception also have feelings of pain and pleasure and consequently appetites, but plants have none of these affections. That the nutrient part is more active when the animal is asleep suggests that sense perception is not required for growth and nutrition.

Part 2

Some animals are endowed with all the modes of sense perception whereas others with only some. No animal when asleep is able to exercise any of the modes of sense perception. Each sense has something peculiar such as seeing or hearing, and something common whereby the person perceives that he is seeing or hearing. This common and controlling sensory activity chiefly subsists in association with the sense of touch, for the sense of touch can exist apart from all the other senses, but none of the other senses can exist apart from the sense of touch. As all animals are endowed with the sense of touch, they are all capable of waking and sleeping.

There are several types of causes, namely, the final, efficient, material, and formal. The final cause of sleep is the conservation of animals, which cannot continually be moving. The exercise of sense perception or of thought is the highest end for any animal, and this implies that (1) the waking state is the highest end for any animal, (2) sleep belongs of necessity to every animal.

As has already been demonstrated in another work, controlling sense perception originates in the same part of the organism in which originates movement. This locus of origination is one of three determinate loci, namely, that which lies midway between the head and the abdomen. In sanguineous animals, this corresponds to the region of the heart.

Part 3

In sanguineous animals food ultimately turns into blood. Blood is contained in the veins, which originate from the heart. Sleep arises from the evaporation attendant upon the process of nutrition. The matter evaporated is hot and rising. Once it has risen to the brain, which is the coolest part of the body, it condenses and falls back down again to the region of the heart, resulting in sleep and then in fantasy. Thus sleep-inducing substances produce a feeling of heaviness in the head, as do fatigue, illness, and extreme youth. Awakening occurs once digestion is complete and the finest and purest blood, which is found in the head, has been separated from the thickest and most turbid blood, which is found in the lower extremities. Sleep resembles epilepsy in that it involves a kind of seizure that paralyses the primary sense organ and prevents it from actualising its powers. However, sleep is only one form of impotence of the perceptive faculty, which can also be rendered impotent by unconsciousness, asphyxia, and swooning.

De Insomniis

Part 1

Sense perception and intelligence are the only faculties by which knowledge is acquired. As no animal when asleep is able to exercise any of the modes of sense perception, it may be concluded that it is not by sense perception that dreams are perceived. But neither is it only by opinion or intelligence, for in dreams it is asserted not only that some approaching object is a man or a horse, which is an exercise of opinion, but also that the object is white or beautiful, which requires at least some element of sense perception. In dreams as in waking moments, it is common to reason about that which is perceived – that is, to think something else over and above the dream presentation – and this too is an exercise of opinion.

The faculty which produces illusory effects during waking moments is identical with that which produces them during sleep. The sun may appear to be only one foot wide, but this illusion does not occur without actually seeing or otherwise perceiving something real. Even to see wrongly or to hear wrongly can only happen upon seeing or hearing something real. It has been assumed that sleep implies an absence of sense perception; it may be true that the dreamer perceives nothing, yet it may be false that his faculty of sense perception is unaffected. Thus, the senses may provide impulses to the primary sense organ, though not in the same manner as during waking moments.

Let us then assume that sleeping and dreaming both appertain to the same faculty of sense perception. In On the Soul, it has been established that the faculty of presentation is identical with that of sense perception, even though the essential notion of a faculty of presentation is different from that of a faculty of sense perception. Since presentation is the movement set up by a sensory faculty upon discharging its function, and since a dream appears to be a presentation, it follows that dreaming is an activity of the faculty of sense perception, but that it belongs to this faculty as a presentative.

Part 2

The affection due to objects that produce sense perception is present in the organ of sense perception not only when the perceptions are actualised, but even when they have departed. Just as with projectiles moving in space, the movement continues even though that which set up the movement is no longer in contact with that which is being moved. So it is that, if one turns the gaze from sunlight to darkness, one sees nothing owing to the light still subsiding in the eyes. Also, if one looks a long time at one colour, that to which one transfers the gaze appears to be of that same colour. There are many other such phenomena.

As demonstrated by the case of mirrors, the sensory organs are acutely sensitive to even a slight qualitative difference in their objects. The eye in seeing is affected by the object seen, but it also produces a certain effect upon it. For instance, if a woman chances during the menstrual period to look into a highly polished mirror, the surface of it will grow cloudy with a blood-coloured haze. This stain is very hard to remove from a new mirror, but easier to remove from an older mirror. Thus, it is clear that stimulatory motion is set up even by slight differences, and that sense perception is quick to respond to it; and further that the eye is not only affected by its object, but also produces a certain effect upon it.

Let us then assume that the impressions of an object of perception remain even after the object has departed, and, further, that they are themselves objects of perception. Let us also assume that sense perception can be deceptive in the presence of emotions such as fear, desire, and anger. This explains why people in the delirium of fever sometimes think that they see animals on their chamber walls. The cause of such illusions is that the faculty by which the controlling sense judges is not the same as that by which it perceives. False judgements arise because appearances result not only from its object stimulating a sense, but also from the sense alone being stimulated in the same manner as by the object. Thus, to a person in a sailing ship it may appear that the land is moving, when in reality it is the person’s eye that is being moved by the ship.

Part 3

During sleep, owing to the inaction of the particular senses, stimulatory movements from causes within the body present themselves with greater impressiveness.

Like the eddies which are being ever formed in rivers, so stimulatory movements are each a continuous process, often remaining as they first started, but often being broken into other forms by collisions with obstacles. This explains why dreams do not occur immediately after a meal or in infants; in each of these cases, the violence of internal movement is such as to obliterate any sensory impressions, or to distort them into unhealthy dreams. Once food has been digested, the blood becomes calm and pure once again. This enables stimulatory movements to be preserved in their integrity and a clear image to be presented.

Not every presentation that occurs during sleep is necessarily a dream, for it is possible for the sleeping person to dimly and, as it were, remotely perceive light and sound and other external stimuli. Indeed, it is quite possible that, of waking or sleeping, while the one is present the other is also present in a certain way. Such occurrences should not be called a dream, and neither should the true thoughts, as distinct from the mere presentations, that occur during sleep.

De Divinatione per Somnum

Part 1

The divination that takes place in sleep, and that is said to be based on dreams, cannot be dismissed lightly. At the same time, it cannot easily be accounted for. It is claimed that the sender of such dreams is God, but this is difficult to reconcile with the fact that those to whom he sends them are not the best and wisest, but merely the commonplace. At the same time, none of the other possible causes appear probable.

Divinatory dreams must be regarded either as tokens or as causes of the events that they contain, or else as coincidences, or as more than one, and possibly all, of these. Even scientific physicians say that one should pay attention to dreams, and it is reasonable even for speculative philosophers to share in this belief. For the movements which occur in the body during waking moments are generally eclipsed by waking movements. However, this is not the case during sleep, when even trifling movements seem considerable. For instance, dreamers fancy that they are affected by thunder and lightning when there is but a faint ringing in their ears, or that they are walking through a fire when there is but a slight warming over certain parts of their body. As the beginnings of all events are small, the beginnings of diseases or other bodily affections are more evident in sleep than in waking moments.

Neither is it improbable that some dreams are the causes of the actions which they contain. It is clear enough that potential or actual waking actions often shape our dreams, in which they may even be played out or repeated. In such cases, the daytime movements have paved the way for the dream movements. Conversely, it must be that dream movements can pave the way for daytime movements, and thus that dreams can shape our waking actions.

That having been said, most so-called prophetic dreams are mere coincidences, particularly if they are extravagant or remote in place or if the dreamer does not have any initiative over the dream content. In waking moments, it is common for a person to mention a thing and then to find that it comes to pass. Why, then, should such an occurrence not also be common in sleep?

Part 2

Dreams occur in inferior people and in certain of the lower animals, and are neither sent by God nor designed for the purpose of divination. However, they do have a divine aspect as Nature is divinely planned, though not itself divine. According to the gambler’s maxim, ‘If you make many throws your luck must change’, and so it is that people who are garrulous and excitable and who have many dreams are likely to have their dreams fulfilled.

That many dreams have no fulfilment is only to be expected, since another more influential movement could mean that that which is about to happen is not in every case that which is now happening. These beginnings from which no consummation follows are nonetheless real beginnings; they constitute natural tokens of certain events, albeit events that did not come to pass.

Some divinatory dreams do not involve the beginnings of future events, but are extravagant in time, place, or content, or are not extravagant but the dreamer does not have any initiative over the dream content. They may be a simple matter of coincidence, or they may result from distant movements that are perceptible to the sleeping soul. Such movements are more perceptible at night because then the air is less disturbed, and because people are more sensitive to slight movements during sleep than during waking moments. Divinatory dreams occur in inferior and commonplace people because their minds are derelict or totally vacant and so more conductive of alien movements. Divinatory dreams are particularly common or vivid in people who are liable to derangement because their normal mental movements are beaten off by the alien movements. People often have vivid dreams about those whom they are close to: just as acquaintances recognise and perceive one another from a distance, so they do with regards to the movements respecting one another.

Skilful dream interpretation calls upon the faculty of observing resemblances. Dream presentations are analogous to the forms reflected in water; if the motion in the water is great, the reflection bears no resemblance to its original. In such cases particular skill is required.

Adapted from Aristotle’s Universe, NYP.