On Sleep And Sleeplessness   

either some external movement or else some movement in his own

consciousness. If waking, then, consists in nothing else than the

exercise of sense-perception, the inference is clear, that the

organ, in virtue of which animals perceive, is that by which they

wake, when they are awake, or sleep, 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 in

every case 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 its affection is not an affection of soul

exclusively, and that a soulless body has not the potentiality of

perception. [Thus sleep and waking are not attributes of pure

intelligence, on the one hand, or of inanimate bodies, on the other.]

Now, whereas we have already elsewhere distinguished what are called

the parts of the soul, and whereas the nutrient is, in all living

bodies, capable of existing without the other parts, while none of the

others can exist without the nutrient; it is 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 its potentiality, indeed, and in its

relationships, it is separable.

Likewise it is clear that [of those which either sleep or wake]

there is no animal which is always awake or always asleep, but that

both these affections belong [alternately] 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 should perpetually attach itself to the same animal, e.g.

that some species of animal should be always asleep or always awake,