“Through science, for peace” could be Langevin’s motto in the immediate aftermath of the First World War. There was a spontaneous alliance between the interests of the scholar and those of the pacifist. Doing science already soothed the grudges. This chapter explores the convictions and moral values underlying Langevin’s peace activism. Like many of his contemporary intellectuals, he reflected on the shock of the world war and on the part that science played in it. He consequently forged a mental world centred on the moral value of science and its social and political function as “mother and daughter of democracy” and “elder sister of social justice”.