During the late 1940s and early 1950s, David Riesman and Hannah Arendt were engaged in an animated discussion about the meaning and character of totalitarianism. Their disagreement reflected, in part, different experiences and dissonant intellectual backgrounds. Arendt abhorred the social sciences, finding them pretentious and obfuscating. Riesman, in contrast, abandoned a career in law to take up the sociological vocation, which he combined with his own heterodox brand of humanistic psychology. This article delineates the stakes of the Arendt Riesman debate by examining Arendt’s critique of social science and Riesman’s defence of a sociological interpretation of totalitarianism. In addition, the article argues that Arendt’s theory of totalitarianism misdescribed the nature of Nazi and Bolshevik societies in ways that damaged her political account more generally. Riesman intuited that weakness and, as the following article shows, modern historical research has confirmed it.