  In his Principia, Newton had suggested that the absolute motion of bodies is to be defined relative to absolute space and time, and to be discovered by its properties, causes and  effects.[30]  Leibniz, as we have just seen, opposed such a view, holding instead that true motions are to be defined with respect to the active forces that he took to be inherent in truly moving bodies. This disagreement between Newton and Leibniz over the nature of true motion surfaced more explicitly in their disagreement over the nature of space and time in the Leibniz-Clarke correspondence. In the five letters he managed to write before his death, Leibniz succeeded in articulating not only his reasons for opposing what he took to be Newton’s conception of absolute space and time, but also sketching an alternative picture according to which they are to be understood as abstract systems of relations. 