Today we deliberately refer to social sciences in the plural. For much of the nineteenth century, however, writers more characteristically spoke of social science or la science sociale in the singular. Although there was perhaps as little consensus then as now on either the meaning of ‘social’ or the methods of its ‘science(s)’, there was an often unspoken agreement about the relationship of social science to politics: la science sociale would provide the master plan for a new political order. My purpose in this essay is not to canvas all the uses of social science as political blueprint, but rather to reconsider some key debates about the relationship of social science to political argument in France and England from the French Revolution, when the term science sociale became current, to the 1880s, when ‘positivism’ had come to prevail on both sides of the Channel. To this end, I will contrast the reach and resonance of the idea of ‘social science’ in two political milieux.