On The Motion Of Animals   

generally speaking accompanied by a definite change of temperature

in the body. One may see this by considering the affections. Blind

courage and panic fears, erotic motions, and the rest of the corporeal

affections, pleasant and painful, are all accompanied by a change of

temperature, some in a particular member, others in the body

generally. So, memories and anticipations, using as it were the

reflected images of these pleasures and pains, are now more and now

less causes of the same changes of temperature. And so we see the

reason of nature's handiwork in the inward parts, and in the centres

of movement of the organic members; they change from solid to moist,

and from moist to solid, from soft to hard and vice versa. And so when

these are affected in this way, and when besides the passive and

active have the constitution we have many times described, as often as

it comes to pass that one is active and the other passive, and neither

of them falls short of the elements of its essence, straightway one

acts and the other responds. And on this account thinking that one

ought to go and going are virtually simultaneous, unless there be

something else to hinder action. The organic parts are suitably

prepared by the affections, these again by desire, and desire by

imagination. Imagination in its turn depends either upon conception or

sense-perception. And the simultaneity and speed are due to the

natural correspondence of the active and passive.

However, that which first moves the animal organism must be

situate in a definite original. Now we have said that a joint is the

beginning of one part of a limb, the end of another. And so nature

employs it sometimes as one, sometimes as two. When movement arises

from a joint, one of the extreme points must remain at rest, and the

other be moved (for as we explained above the mover must support

itself against a point at rest); accordingly, in the case of the

elbow-joint, the last point of the forearm is moved but does not

move anything, while, in the flexion, one point of the elbow, which

lies in the whole forearm that is being moved, is moved, but there