In the context of nationalizing, secularizing or Kemalist states, analyses of Islamist movements are often thrown back on notions of traditionalism or atavism. In a related vein, for certain social theorists writing on modernity, the uniqueness of the West is clarified through an imaginative [mis]interpretation of other cultures or civilizations. Too often, however, the apparent gains in Western self-insight reflect an ‘inability to constitute oneself without excluding the other’ (Cornelius Castoriadis). Ironically Castoriadis himself, in a project we might term an ethnography of the West (see his writings on ecology, capitalism, rationality, contemporary culture, racism, Greek philosophy/history, and the environment) is prone to the same vice, especially in his identification of the West as the sole autonomous society. This article argues that in the shariainstitution of a legal autonomy, Islam and Islamism alike demonstrate an affinity with modernity as defined by Castoriadis. In the light of this Islamist autonomy, it concludes that Castoriadis’ vision of modernity as a struggle between opposed imaginaries of autonomy and rational mastery needs reformulating.