24. As to movement of displacement, we may ask if ascending be the
opposite of descending, in what the circular movement differs from the
rectilinear movement, what difference obtains in throwing an object
at the head or at the feet. The difference is not very clear, for in
these cases the motive power is the same. Shall we say that there is
one power which causes raising, and another that lowers, especially
if these movements be natural, and if they be the result of lightness
or heaviness? In both cases, there is something in common, namely,
direction towards its natural place, so that the difference is derived
from exterior circumstances. Indeed, in circular and rectilinear
movement, if someone move the same object in turn circularly and
in a straight line, what difference is there in the motive power?
The difference could be derived only from the figure (or outward
appearance) of the movement, unless it should be said that the
circular movement is composite, that it is not a veritable movement,
and that it does not produce any change by itself. In all of these
cases, the movement of displacement is identical, and presents only
adventitious differences.