This paper studies the ‘liberal solidarity’ of Léon Bourgeois through two fundamental and interlinked ‘liberal solidarist principles’: (i) social debt and (ii) the quasi-contract. No real work has been proposed in economics in English in the context of Bourgeois’s work. It is this shortfall that we wish to fill in this article by restoring a little-known ancient thought and proposing an initial contribution to investigating future avenues of research in institutional economics. The solidarism of the Third French Republic allows us to rethink social democratic liberalism from an open and ongoing view of institutions as the driving force behind both individual rights and the collective order. Bourgeois solidarism is an interesting doctrine for rethinking from a moral perspective the role of human duty in economics and, more largely, in society.