Aristotle on sleep and dreams 177
the other hand, does not know fatigue and the harder we exercise our
intellectual faculty, the better it functions.^22
The way in which Aristotle arrives at these views is largely theoretical and
bya priorireasoning. Sleep, he argues in chapter 1 ofOn Sleep and Waking,
is the opposite of waking; and since waking consists in the exercise of the
sensitive faculty, sleep must be the inactivity of this faculty. Sleep affects all
animals, because sensation is characteristic of animals. Plants do not sleep,
because they have no perception. In fact, sleep is nothing but a state of what
Aristotle elsewhere calls ‘first entelechy’,^23 a state of having a faculty without
using it, which may be beneficial in order to provide rest to the bodily parts
involved in its exercise. Furthermore, Aristotle is characteristically keen to
specify that sleep is a particular kind of incapacitation of the sense faculty
as distinct from other kinds of incapacitation, such as faint and epileptic
seizure ( 456 b 9 – 16 ). He also applies his explanatory model of the four causes
(which he reminds us of in 455 b 14 – 16 ) to the phenomenon of sleep, listing
its formal, final, material and efficient causes, and leading up to two com-
plementary definitions stating the material and the formal cause of sleep:
the upward movement of the solid part of nutriment caused by innate heat, and its
subsequent condensation and return to the primary sense organ. And the definition
of sleep is that it is a seizure of the primary sense organ which prevents it from
being activated, and which is necessary for the preservation of the living being;
for a living being cannot continue to exist without the presence of those things
that contribute to its perfection; and rest (anapausis) secures preservation (s ̄ot ̄eria).
( 458 a 25 – 32 )By contrast, there is little consideration, let alone evidence of systematic
gathering and interpreting, of empirical evidence to back up the theory
arrived at. It is true that, in the course of his argument, Aristotle occasionally
refers to empirical observations, or at least he makes a number of empirical
claims, which can be listed as follows:
1. Most animals have their eyes closed when they sleep ( 454 b 15 ff.).
2. Nutrition and growth are more active in sleep than in the waking state ( 455 a
1 – 2 ).
3. All animals have a sense of touch ( 455 a 6 ).
4. In fainting fits, people lose sensation ( 455 b 6 ).
5. Those who have the veins in the neck compressed become unconscious
( 455 b 7 ).
6. Breathing and cooling take place in the heart ( 456 a 5 ).
7. Insects that do not respire are seen to expand and contract ( 456 a 12 ).(^22) De an. 429 a 30 –b 6. (^23) De an. 412 a 25 – 6.