On ten loose handwritten folios dating back from April 1679, Leibniz gradually devised, in the course of three days, a full-blown theory of thought that nonetheless remained unpublished and still has received little attention from scholars. Conceiving of affectūs as the driving forces that set the mind in motion from one thought to another and passions as the inertia opposing such movement, this manuscript results in a systematic psychology understood as a dynamics of thoughts modelled on the mechanical laws of motion for solid bodies. Delving into Leibniz‘s working papers to witness the unfolding of his thoughts, I propose to pay attention to the many intellectual operations that paved the way for his metaphysics. From his reading notes on Descartes to his syllogistically redefining a set of concepts and propositions, Leibniz here defines an affective theory of cognition and sets the first foundations of a combinatorial ontology: his socalled scientia generalis. Focusing on the material practices that govern his use of paper, I would like to show that Leibniz‘s conceptual mechanization of cognition is materially dependent on a practical automation of reasoning reduced to a propositional calculus on paper. Eventually, this contribution is a plea for a media-historical reading of Leibniz‘s working papers.