328 since it is always weakened. So it is true that the force and the heaviness will pass through ratios like 2/1, 3/2, 3/4 and countless others; but that they remain in any one of them for a certain time is false and impossible: thus at some time they even arrive at equality, but they do not remain in equality. Since this is so, and since local upward and downward motion is a consequence of that alterative motion of the transformation from light by accident into heavy per se, in such a way that that upward motion follows from an excess of impressed force, downward from its deficiency, and rest from equality; since equality does not last for a length of time, it is necessary that the rest does not last.
Aristotle thought (as can be seen in De Caelo I, text #88 [277a 27-33]) {1}, not only that natural motion is always accelerated until the mobile has reached its proper place, but also that, if the motion could continue without end, the heaviness of the mobile and the swiftness of its motion would also be increased without end. For, in trying to show that things that are moved are carried towards a certain definite place, he has written this: If earth, while it is moved downward, were not moved towards a definite place, but without end, its heaviness and its speed would also be increased without end; but a heaviness without end and a speed without end cannot be given; hence what is carried downward is not moved without end. This, then, is Aristotle's opinion: now we will make it manifest that the truth is diametrically opposed to this; namely, we will show that the speed is not always increased, and that even if it were always increased and the motion could be extended without end, [the mobile] would not necessarily attain a heaviness without end and a swiftness without end.
Thus as to what concerns the first point, anyone will easily understand from the things that have been written above {1}, having grasped the cause of the acceleration of natural motion at the end, why such an acceleration must finally cease. For since the mobile is accelerated because the contrary force weakens continuously,