How can we make economics genuinely quantitative? This is the question that Gabriel Tarde tackled at the end of his career. His solution was in total contradiction to the dominant views of his time: to quantify the connections between people and goods, you need to grasp “passionate interests.” Capitalism is not a system of cold calculations, but, rather, a constant amplification in the intensity and reach of passions. In a stunning anticipation of contemporary economic anthropology, Tarde’s work traces a patch between the tangle of ideas that have been responsible for so much modern misery: the first between adepts of the Invisible Hand and the devotees of the Visible Hand. In this pamphlet, Latour and Lépinay offer a lively introduction to the work of this forgotten genius of nineteenth-century French social thought.