  [I]f we were to imagine that there are atoms, that is, bodies of  maximal hardness and therefore inflexible, it would follow that there would be a change through a leap, that is, an instantaneous change.  For at the very moment of collision the direction to the motion  reverses itself, unless we assume that the bodies come to rest  immediately after the collision, that is, lose their force; beyond  the fact that it would be absurd in other ways, this contains, again, a change through a leap, and instantaneous change from motion to  rest, without passing through the intermediate steps. (SD II.3/AG  132)