How Leibniz Read Pascal’s Geometry

Between 1675 and 1676, while in Paris, Gottfried Wilhelm Leibniz (1646–1716) got privileged access to some geometric manuscripts from late Blaise Pascal’s hand. Although said manuscripts are not extant, Leibniz’s reading notes were preserved, together with his personal papers in Hannover, under the heading “Pascaliana.” The mathematical content of these notes and the influence they had on Leibniz’s later work are quite known nowadays. At the crossroads of the history of ideas, historical epistemology, and material history, this contribution looks at Leibniz’s Pascaliana through the prism of their materiality—format, layout, organization, corrections, and additions—, and the practices it betrays—copying, commenting, excerpting. Parallel to knowing what Leibniz read in Pascal, this perspective allows us to shed light on how Leibniz read Pascal, on the very intellectual-material operations that allowed him to incorporate a foreign thought and, eventually, start developing his own.