Now
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.
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