 The Principle of Continuity also played an important underlying role in Leibniz’s mathematical work, especially in his development of the infinitesimal calculus. Leibniz’s essays Nova Methodus of 1684 and De Geometria Recondita of 1686 may be said to represent the official births of the differential and integral calculi, respectively. His approach to the calculus, in which the use of infinitesimals, plays a central role, has combinatorial roots, traceable to his early work on derived sequences of numbers. Given a curve determined by correlated variables \(x, y\), he wrote \(\Dx\) and \(\Dy\) for infinitesimal differences, or differentials, between the values \(x\) and \(y\): and \(\Dy/\Dx\) for the ratio of the two, which he then took to represent the slope of the curve at the corresponding point. This suggestive, if highly formal procedure led Leibniz to evolve rules for calculating with differentials, which was achieved by appropriate modification of the rules of calculation for ordinary numbers.