Between 1671 and 1716, the German mathematician Gottfried Wilhelm Leibniz attempted to design a reckoning machine. Working closely with the Parisian clockmaker Monsieur Ollivier, Leibniz eventually failed at producing a working prototype. After the rediscovery of the machine’s last model in 1879, this failure was usually attributed either to their time’s technology—supposedly not advanced enough for Leibniz’s genius—, or to communication problems between the mathematician and the artisan. Along his correspondence, Leibniz left a bounteous collection of texts and schematics dedicated to the technical specification of his envisioned machine. In his own words, this “arithmetic instrument” aimed at “transplanting” human mind into “inanimate matter”—thus performing an operation of re-mediation. Following Leibniz’s own metaphysics, it could be argued that his paper arithmetic would therefore be losslessly translated into a brass machine, from one medium to another. Though, this leaves wide open a gap between technology and mathematics, as well as between media, that this paper aims at bridging. Adopting a material perspective on Leibniz’s working papers and their relation to his metaphysics, as well as to Ollivier’s prototype, I will try to shed light on a few steps of a long process of re-mediating reckoning into a mechanical device.