The paper presents an analysis of the cosmological systems of three socialist theoreticians of the 19th century : Saint-Simon, Charles Fourier, and Auguste Blanqui. Its aim, in taking at face value each thinker’s project of constructing a system accounting for both the natural and the social world, is to show how, in each case, such theoretical constructions were part and parcel of the political projects. These authors conceive of cosmologies, and aspire to scientific legitimacy for these constructions ; these cosmologies can be analysed as discourses regarding the natural order, and are supposed to legitimate the prescriptions concerning social organisation and the modalities of the exercise of power that the authors develop elsewhere, or simultaneously.