 It was noted above that Cassirer’s early historical works interpret the development of modern thought as a whole (embracing both philosophy and the sciences) from the perspective of the philosophical principles of Marburg neo-Kantianism, as initially articulated in [Cohen 1871]. On the “genetic” conception of scientific knowledge, in particular, the a priori synthetic activity of thought – the activity Kant himself had called “productive synthesis” – is understood as a temporal and historical developmental process in which the object of science is gradually and successively constituted as a never completed “X” towards which the developmental process is converging. For Cohen, this process is modelled on the methods of the infinitesimal calculus (in this connection, especially, see [Cohen 1883]). Beginning with the idea of a continuous series or function, our problem is to see how such a series can be a priori generated step-by-step. The mathematical concept of a differential shows us how this can be done, for the differential at a point in the domain of a given function indicates how it is to be continued on succeeding points. The differential therefore infinitesimally captures the rule of the series as a whole, and thus expresses, at any given point or moment of time, the general form of the series valid for all times,