On The Motion Of Animals   

1

ELSEWHERE we have investigated in detail the movement of animals

after their various kinds, the differences between them, and the

reasons for their particular characters (for some animals fly, some

swim, some walk, others move in various other ways); there remains

an investigation of the common ground of any sort of animal movement

whatsoever.

Now we have already determined (when we were discussing whether

eternal motion exists or not, and its definition, if it does exist)

that the origin of all other motions is that which moves itself, and

that the origin of this is the immovable, and that the prime mover

must of necessity be immovable. And we must grasp this not only

generally in theory, but also by reference to individuals in the world

of sense, for with these in view we seek general theories, and with

these we believe that general theories ought to harmonize. Now in

the world of sense too it is plainly impossible for movement to be

initiated if there is nothing at rest, and before all else in our

present subject- animal life. 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