The Leibniz differential and the Perron–Stieltjes integral

We implement Leibniz's idea about the differential as the length of an infinitesimally small elementary interval (a monad) in the form satisfying modern standards of rigor. The concept of sequential differential introduced in this paper is shown to be in good alignment with the standard convention of the integral calculus. As an application of this concept we simplify and generalize the construction of the Perron–Stieltjes integral.