  Leibniz’s argument here is that as the sizes of the bodies B and C change continuously from inequality to equality the  effects of that change should be continuous as well. Descartes’s  rules of impact, however, would have an infinitesimal change in input – a change from B’s being infinitesimally bigger than  C, to C’s being equal in size to B – result in a leap of output – from C rebounding while  B remains stationary, to both B and C  rebounding. Leibniz maintains that Descartes’s rules of impact must  therefore be false since they violate the principle of continuity. 