176 Aristotle and his school
(either perceptual or intellectual, and either ‘normal’ or extraordinary) in
sleep. But this question is not explicitly raised, and his relevant remarks
are scattered, nuanced and complicated, so that an answer unfolds only
gradually through the continuous discussion in the three works; and it is
not free from apparent contradictions. InOn Sleep and Waking, Aristotle
begins by defining sleep negatively as the inability of the sense faculty to
be activated (adunamia tou energein, 454 b 5 , 458 a 29 ). Sleep is said to
be a ‘fetter and immobilisation’ (desmos kai akin ̄esia, 454 b 10 ) an ‘inac-
tivity’ (argia) or ‘incapacitation’ (adunamia) of the sensitive faculty ( 455 b
3 ff.). It is a state in which the vegetative part of the soul gains the upper
hand ( 455 a 1 – 2 ), and it is caused by various physiological processes that
are connected with the digestion of food (such as heating, cooling, evapo-
ration of food, and sifting of the blood).^19 There is no sensation in sleep,
he says, because the central sense-organ, thekurion aisth ̄et ̄erion, which is
located in the heart, is affected by these processes and thus incapacitated,
and as a result of this the peripheral sense-organs (eyes, ears, nose, etc.)
cannot function either ( 455 a 13 ff.). Whether these physiological processes
also affect the ability to think and the operations of the intellectual part of
the soul, is a question which Aristotle does not address explicitly. Strictly
speaking, since Aristotle’s supreme intellectual faculty, thenous, is said to
be incorporeal and not to require simultaneous perception in order to be
active,^20 there is, at least in principle, no reason why we should not be able
to think while being asleep.^21 There are a few hints to this in the text, for
example inInsomn. 459 a 6 – 8 and 462 a 29 – 30 , which speak of an activity of
‘judgement’ (doxa) and of the presence of ‘true thoughts’ (al ̄etheis ennoiai)
in sleep, but it remains vague (see below).
Aristotle’s negative definition of sleep does not, however, imply a negative
evaluation of this ‘affection’ (pathos). Sleep is a good thing and serves a
purpose, for it provides rest (anapausis) to the sense-organs, which would
otherwise become overstretched, since they are unable to be active without
interruption ( 454 a 27 , 455 b 18 ff.). Here, again, one may note a difference
compared with thinking; for one of the differences between perception and
thinking, according to Aristotle, is that perception cannot go on forever,
indeed if we overstretch our sense-organs, we damage them; thinking, on
(^19) For a discussion of Aristotle’s physiological explanation of sleep see Wiesner ( 1978 ).
(^20) De an. 430 a 17 – 18 , 22 – 3 ;Gen. an. 736 b 28 – 9 ; see also ch. 7 below.
(^21) InDe an. 429 a 7 – 8 Aristotle mentions the possibility that the intellect (nous) may be ‘overshadowed’
(epikaluptesthai) by sleep, but it is unclear from this passage whether this is always the case in sleep
or only in exceptional circumstances.