For if one of the parts of an animal
be moved, another must be at rest, and this is the purpose of their
joints; animals use joints like a centre, and the whole member, in
which the joint is, becomes both one and two, both straight and
bent, changing potentially and actually by reason of the joint. And
when it is bending and being moved one of the points in the joint is
moved and one is at rest, just as if the points A and D of a
diameter were at rest, and B were moved, and DAC were generated.
However, in the geometrical illustration, the centre is held to be
altogether indivisible (for in mathematics motion is a fiction, as the
phrase goes, no mathematical entity being really moved), whereas in
the case of joints the centres become now one potentially and
divided actually, and now one actually and divided potentially. But
still the origin of movement, qua origin, always remains at rest
when the lower part of a limb is moved; for example, the elbow
joint, when the forearm is moved, and the shoulder, when the whole
arm; the knee when the tibia is moved, and the hip when the whole leg.
Accordingly it is plain that each animal as a whole must have within
itself a point at rest, whence will be the origin of that which is
moved, and supporting itself upon which it will be moved both as a
complete whole and in its members.
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