On Sleep And Sleeplessness   

given, all is quite clear regarding those also. For, when the sense

organ which controls all the others, and to which all the others are

tributary, has been in some way affected, that these others should

be all affected at the same time is inevitable, whereas, if one of the

tributaries becomes powerless, that the controlling organ should

also become powerless need in no wise follow.

It is indeed evident from many considerations that sleep does not

consist in the mere fact that the special senses do not function or

that one does not employ them; and that it does not consist merely

in an inability to exercise the sense-perceptions; for such is what

happens in cases of swooning. A swoon means just such impotence of

perception, and certain other cases of unconsciousness also are of

this nature. Moreover, persons who have the bloodvessels in the neck

compressed become insensible. But sleep supervenes when such

incapacity of exercise has neither arisen in some casual organ of

sense, nor from some chance cause, but when, as has been just

stated, it has its seat in the primary organ with which one

perceives objects in general. For when this has become powerless all

the other sensory organs also must lack power to perceive; but when

one of them has become powerless, it is not necessary for this also to

lose its power.

We must next state the cause to which it is due, and its quality

as an affection. Now, since there are several types of cause (for we

assign equally the 'final', the 'efficient', the 'material', and the

'formal' as causes), in the first place, then, as we assert that

Nature operates for the sake of an end, and that this end is a good;

and that to every creature which is endowed by nature with the power

to move, but cannot with pleasure to itself move always and

continuously, rest is necessary and beneficial; and since, taught by

experience, men apply to sleep this metaphorical term, calling it a

'rest' [from the strain of movement implied in sense-perception]: we

conclude that its end is the conservation of animals. But the waking