2
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.
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