Starting from Marx's unpublished excerpts from the Essai sur la Nature du Commerce én Général by Richard Cantillon, this paper traces the impact that the reading of the Essai exercised on both Marx's assessment of Cantillon and his own view of the origins of political economy. It is shown that closer acquaintance of Marx with Quesnay's Tableau Ēconomique and with Cantillon's Essai occurred almost simultaneously, and this joint discovery led him to supplement his value-centred view of early classical economists with a new circular flow, or macroeconomic perspective within which Cantillon's role was recognised to be crucial. Marx's analysis of the Cantillon–Quesnay connection is supported by a review of an early contribution to Cantillon scholarship by the Soviet economist Alexandra Eidelnant.