On The 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.

The doctrine of the Pythagoreans seems to rest upon the same

ideas; some of them declared the motes in air, others what moved them,

to be soul. These motes were referred to because they are seen

always in movement, even in a complete calm.

The same tendency is shown by those who define soul as that which

moves itself; all seem to hold the view that movement is what is

closest to the nature of soul, and that while all else is moved by

soul, it alone moves itself. This belief arises from their never

seeing anything originating movement which is not first itself moved.

Similarly also Anaxagoras (and whoever agrees with him in saying

that mind set the whole in movement) declares the moving cause of

things to be soul. His position must, however, be distinguished from