MOVEMENT OF DISPLACEMENT IS SINGLE.
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.