On The Motion Of Animals   

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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But the point of rest in the animal is still quite ineffectual

unless there be something without which is absolutely at rest and

immovable. Now it is worth while to pause and consider what has been

said, for it involves a speculation which extends beyond animals

even to the motion and march of the universe. For just as there must

be something immovable within the animal, if it is to be moved, so

even more must there be without it something immovable, by

supporting itself upon which that which is moved moves. For were

that something always to give way (as it does for mice walking in

grain or persons walking in sand) advance would be impossible, and

neither would there be any walking unless the ground were to remain

still, nor any flying or swimming were not the air and the sea to

resist. And this which resists must needs be different from what is

moved, the whole of it from the whole of that, and what is thus

immovable must be no part of what is moved; otherwise there will be no

movement. Evidence of this lies in the problem why it is that a man

easily moves a boat from outside, if he push with a pole, putting it

against the mast or some other part, but if he tried to do this when

in the boat itself he would never move it, no not giant Tityus himself

nor Boreas blowing from inside the ship, if he really were blowing

in the way painters represent him; for they paint him sending the

breath out from the boat. For whether one blew gently or so stoutly as

to make a very great wind, and whether what were thrown or pushed were