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Whether sleep is necessary for an animal.
"Let us discuss sleep," etc. First, one asks whether sleep is necessary for an animal.
1. It seems not. Act occurs by virtue of form and affect [ pas-sio] by virtue of matter. Therefore, to the extent that one has more form, then to that extent one can act more. But an animal has more of form than fire does. Therefore, since fire can act continuously and does not require rest, an animal can perceive all the more without an intervening period of rest.
2. Moreover, the nobler the form is, the more it is capable of numerous operations. And the more superior a power is, the less is it given to fatigue. But an animal power is higher than a vital or natural power. Since then a vital or natural power does not require rest, like sleep, then neither does the animal power. But sleep is only necessary to provide rest for an animal power; therefore, etc.
3. Furthermore, sleep occurs in an animal, according to Avicenna, on account of nourishment, because during sleep the powers turn inward so that the nourishment can be digested better. And this is why the exterior parts are insensible, as it were, during sleep. But plants receive nourishment just as animals do. If sleep is necessary for animals on account of this cause, then it will be necessary too for plants, which is false. Therefore, the first is false; therefore, etc.
The opposite is evident to sense, because something is necessary if a thing cannot exist without it. But an animal cannot exist without sleep. Therefore, etc.
One must respond that sleep is necessary for an animal. And there are many causes of this. One stems from the matter, because after nourishment has been received the vapors ascend to the brain and there they are thickened and condensed by the brain's coldness. This is just as we see in the larger world where vapors that rise up from the earth and water ascend by means of the sun's power to the middle layer of the air. In the brain, then, these vapors are next condensed and block the paths of the animal spirits that minister to sense and motion and prevent the animal power from reaching the exterior senses (this is just as clouds placed between the sun and the moon prevent their light from reaching us), and then the exterior senses must desist from their operation. Sleep then occurs, because "sleep is the restraint of the senses," as is said in On Sleep and Waking.
Another cause stems from the end, because every mover, in moving the moved, is fatigued by the continuous motion. But the soul is the principle of moving the body, and, while moving, it is moved in an accidental manner and thus is fatigued. For this reason, rest is necessary to it, and this comes upon it as sleep.
A third cause can be that when two powers are in something, if one ceases then the other is strengthened. Thus, if a human having two eyes loses one, he will see better with the one left to him than he did previously with the other. But an animal that receives a great deal of nourishment, not continuously but all at once, needs, then, its natural power strengthened after the reception of nourishment. But this can only occur when the other powers are at rest, and this is why sleep is necessary. Thus digestion is performed better during sleep than during waking.
1. On to the arguments. To the first, one must respond that power differs in an animal and in an inanimate thing because the former is much nobler. But now it is the case that there are few operations in inanimate things. For example, in fire there is an operation that follows from its substantial form, and that is why it can operate as long as its form remains. But in animate beings there are some operations that relate to well-being that are not consequent on the potency of its form, but come forth from the form itself, with the mediation of its powers. And thus, because they do not follow the form immediately as do the operations of inanimate things, therefore, etc.
Or, it can be said in another way that although the animal may have more of form, it is nevertheless not necessary that it operate this way continuously.
2. To the second argument one must respond that an animal power is one thing, and both the vital and the natural powers are something else. For the animal power is present in a human for the sake of well-being [ bene esse], and this is true at least of the external senses. But the natural and vital powers are for the sake of being [esse], since the natural power exists for the restoration of the moisture that is lost whereas the vital power serves to regenerate the heat and spirit that have been consumed, and since an animal cannot exist without these and since they are nevertheless being lost continually, this is why the vital power (which is in the heart) and the natural power (which is in the liver and in other members) act continuously, whereas the animal power does not.
Another cause can be that the natural power and the vital power are strengthened by their objects, but that this is not the case for the animal power. For the visual power is not strengthened by color, and hearing is not strengthened by sound.
3. To the third argument one must reply that sleep is in the animal not only for the sake of the digestion of nourishment but also for the sake of rest for the senses, which are continually fatigued. And, furthermore, a plant continuously takes in nourishment but does not take in more than it can digest, and this is why sleep is not necessary for a plant. But the animal takes in its nourishment all at once and this is why it needs the digestive power to be strengthened more after receiving nourishment than before doing so, which cannot occur unless some other powers cease to act. This is why, etc.
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