The starting-point of our inquiry is an exposition of those
characteristics which have chiefly been held to belong to soul in
its very nature. Two characteristic marks have above all others been
recognized as distinguishing that which has soul in it from that which
has not-movement and sensation. It may be said that these two are what
our predecessors have fixed upon as characteristic of soul.
Some say that what originates movement is both pre-eminently and
primarily soul; believing that what is not itself moved cannot
originate movement in another, they arrived at the view that soul
belongs to the class of things in movement. This is what led
Democritus to say that soul is a sort of fire or hot substance; his
'forms' or atoms are infinite in number; those which are spherical
he calls fire and soul, and compares them to the motes in the air
which we see in shafts of light coming through windows; the mixture of
seeds of all sorts he calls the elements of the whole of Nature
(Leucippus gives a similar account); the spherical atoms are
identified with soul because atoms of that shape are most adapted to
permeate everywhere, and to set all the others moving by being
themselves in movement. This implies the view that soul is identical
with what produces movement in animals. That is why, further, they
regard respiration as the characteristic mark of life; as the
environment compresses the bodies of animals, and tends to extrude
those atoms which impart movement to them, because they themselves are
never at rest, there must be a reinforcement of these by similar atoms
coming in from without in the act of respiration; for they prevent the
extrusion of those which are already within by counteracting the
compressing and consolidating force of the environment; and animals
continue to live only so long as they are able to maintain this
resistance.
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