ON THE MOTION OF ANIMALS by Aristotle 350 BC

ON THE MOTION OF ANIMALS by Aristotle translated by A. S. L.
Farquharson

Part 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 join