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On Sleep And Sleeplessness   

insects manifestly assume the posture of sleep; but the sleep of all

such creatures is of brief duration, so that often it might well

baffle one's observation to decide whether they sleep or not. Of

testaceous animals, on the contrary, no direct sensible evidence is as

yet forthcoming to determine whether they sleep, but if the above

reasoning be convincing to any one, he who follows it will admit

this [viz. that they do so.]

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, an

inhibition of function, or, as it were, a tie, imposed on

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 as well;

but plants have none of these affections. A mark of this is that the

nutrient part does its own work better when (the animal) is asleep

than when it is awake. Nutrition and growth are then especially

promoted, a fact which implies that creatures do not need

sense-perception to assist these processes.


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 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

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