This article traces the process that enabled the idea of “solidarity,” born in progressive circles during the Bourbon Restoration, to move from the field of the natural and social sciences to the political sphere. This transfer was the work of a Radical prime minister of the Third Republic, Léon Bourgeois, who in 1896 formulated a political doctrine based on the obligations generated by the irrefutable fact of interdependence between all men. Under the name of “solidarism,” he invented a model that was both social and liberal, and likely to oppose unbridled liberalism as well as rising collectivist ideas. It is this democratic-liberal synthesis that is still being sought today through public policies of solidarity, at both the national and the international level.