Now of animals which change their position some move with the
whole body at once, for example jumping animals, others move one
part first and then the other, for example walking (and running)
animals. In both these changes the moving creature always changes
its position by pressing against what lies below it. Accordingly if
what is below gives way too quickly for that which is moving upon it
to lean against it, or if it affords no resistance at all to what is
moving, the latter can of itself effect no movement upon it. For an
animal which jumps makes its jump both by leaning against its own
upper part and also against what is beneath its feet; for at the
joints the parts do in a sense lean upon one another, and in general
that which pushes down leans upon what is pushed down. That is why
athletes jump further with weights in their hands than without, and
runners run faster if they swing their arms; there is in extending the
arms a kind of leaning against the hands and wrists. In all cases then
that which moves makes its change of position by the use of at least
two parts of the body; one part so to speak squeezes, the other is
squeezed; for the part that is still is squeezed as it has to carry
the weight, the part that is lifted strains against that which carries
the weight. It follows then that nothing without parts can move itself
in this way, for it has not in it the distinction of the part which is
passive and that which is active.
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