On The Motion Of Animals   

that which is moved, but whose nature is not to initiate movement,

is capable of being passive to an external force, while that which

initiates movement must needs possess a kind of force and power. Now

experience shows us that animals do both possess connatural spirit and

derive power from this. (How this connatural spirit is maintained in

the body is explained in other passages of our works.) And this spirit

appears to stand to the soul-centre or original in a relation

analogous to that between the point in a joint which moves being moved

and the unmoved. Now since this centre is for some animals in the

heart, in the rest in a part analogous with the heart, we further

see the reason for the connatural spirit being situate where it

actually is found. The question whether the spirit remains always

the same or constantly changes and is renewed, like the cognate

question about the rest of the parts of the body, is better postponed.

At all events we see that it is well disposed to excite movement and

to exert power; and the functions of movement are thrusting and

pulling. Accordingly, the organ of movement must be capable of

expanding and contracting; and this is precisely the characteristic of

spirit. It contracts and expands naturally, and so is able to pull and

to thrust from one and the same cause, exhibiting gravity compared

with the fiery element, and levity by comparison with the opposites of

fire. Now that which is to initiate movement without change of

structure must be of the kind described, for the elementary bodies

prevail over one another in a compound body by dint of

disproportion; the light is overcome and kept down by the heavier, and

the heavy kept up by the lighter.

We have now explained what the part is which is moved when the

soul originates movement in the body, and what is the reason for this.

And the animal organism must be conceived after the similitude of a

well-governed commonwealth. When order is once established in it there

is no more need of a separate monarch to preside over each several

task. The individuals each play their assigned part as it is