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Whether sleep is present in every animal.

Next one asks whether sleep is present in every animal.

1. It seems not. For many animals live the life of a plant, as will be said below. But sleep is not present in plants and therefore is not present in such animals either.

2. Furthermore, sleep is a restraint of the exterior senses. It prevents the influence of the interior powers on the exterior senses. But there are many animals that lack exterior senses, and therefore, etc.

3. Moreover, when the agent is near to the object acted on, it is necessary that this one act and that one be acted upon, just as, when sight is near to a visible object, vision necessarily occurs. But there are many animals that never close their eyes, like fish and the hare and other animals with a ringed body. They therefore see all the time, and as a result they never sleep.

The Philosopher proves the opposite in the text. For he implies that every animal occasionally ceases from its sensory operations; but such a cessation occurs by means of sleep, and therefore, etc.

One must say that sleep and waking are present in every animal. For sleep is the restraint of the sensitive part. And this occurs at some time for every animal because the operation of sense occurs with a corporeal organ mediating it, and the exertion of one operation or the operation of one power yields to the operation of another. This is evident because when someone intensely imagines something, he does not perceive the things which are offered to sense. This is clear in very studious people who do not show interest in corporeal pleasures. But the sensitive part is very active in a waking state. At such a time, therefore, the natural power yields, and this is why it is necessary at other times that the sensitive operation should yield to some extent and the natural operation be intensified. And sleep therefore comes upon every animal so that the sensitive powers might rest and the nutritive powers work more freely.

1. On to the arguments. To the first, one must respond that some animals are said to live the life of a plant because they are immobile after the manner of plants and they share much in common with plants. But this sharing exists among the powers of the vegetative part, for which sleep is not suitable. But every animal above the plant level has sensation, and this is why sleep comes upon animals, although not upon plants.

2. To the second, one must respond that sleep is the restraint not only of the exterior sense but also of the common sense that is generally found in every animal.

3. To the third argument one must reply that some animals live in a dense, fluid element, like water, and they therefore have hard eyes, so that they will not be harmed, and on account of this these animals do not need eyelids or a covering for the eyes. As a result, they sleep with their eyes open. Thus, although the eyes are not covered, nevertheless the visual power can be withdrawn or directed away from such eyes. There is another reason regarding the hare, namely, that nature is very solicitous toward the preservation of the individual. This is why when someone approaches a terrible place or a place in which something terrible dwells, nature frequently causes a numbness and a shiver and this is so that the individual will be protected from the terror. This is just what frequently happens when someone encounters his enemy or a wolf. On, then, to the proposition. Because the hare is a melancholy animal and very timid, nature permits its eyes to remain open so that it will perceive better if it encounters something harmful.

Or one can say that this is due to the shortness of its eyelids, because it naturally has short ones for the reason already cited, namely, on account of impediments, so that it may perceive those things even while sleeping and waking, etc.

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