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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。
2
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
simple sense any particular sensory faculty whatever； it follows
that in the state called sleep the same affection must extend to all
the special senses； because； if it attaches itself to one of them
but not to another； then an animal while asleep may perceive with
the latter； but this is impossible。
Now； since every sense has something peculiar； and also something
common； peculiar； as； e。g。 seeing is to the sense of sight； hearing to
the auditory sense； and so on with the other senses severally； while
all are accompanied by a common power； in virtue whereof a person
perceives that he sees or hears （for； assuredly； it is not by the
special sense of sight that one sees that he sees； and it is not by
mere taste； or sight； or both together that one discerns； and has
the faculty of discerning； that sweet things are different from
white things； but by a faculty connected in common with all the organs
of sense； for there is one sensory function； and the controlling
sensory faculty is one； though differing as a faculty of perception in
relation to each genus of sensibles； e。g。 sound or colour）； and
since this ＇common sensory activity＇ subsists in association chiefly
with the faculty of touch （for this can exist apart from all the other
organs of sense； but none of them can exist apart from it…a subject of
which we have treated in our speculations concerning the Soul）； it
is therefore evident that waking and sleeping are an affection of this
＇common and controlling organ of sense…perception＇。 This explains
why they belong to all animals； for touch ＇with which this common
organ is chiefly connected＇； alone； ＇is common＇ to all ＇animals＇。
For if sleeping were caused by the special senses having each and
all undergone some affection； it would be strange that these senses；
for which it is neither necessary nor in a manner possible to
realize their powers simultaneously； should necessarily all go idle
and become motionless simultaneously。 For the contrary experience；
viz。 that they should not go to rest altogether； would have been
more reasonably anticipated。 But； according to the explanation just
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
state is for an animal its highest end； since the exercise of
sense…perception or of thought is the highest end for all beings to
which either of these appertains； inasmuch as these are best； and
the highest end is what is best： whence it follows that sleep
belongs of necessity to each animal。 I use the term 'necessity' in its
conditional sense； meaning that if an animal is to exist and have
its own proper nature； it must have certain endowments； and； if
these are to belong to it； certain others likewise must belong to it
＇as their condition。＇
The next question to be discussed is that of the kind of movement or
action； taking place within their bodies； from which the affection
of waking or sleepi