  In addition to attacking Descartes’s laws of motion on the basis of  the principle of the equality of cause and effect, Leibniz also  maintains that they would violate another “metaphysical”  principle, namely, the principle of continuity according to  which continuous changes in inputs should lead to continuous changes  in   outputs.[23]  Thus, for example, in a letter to Malebranche of July 1687, Leibniz  writes: