The contemporary idea of ‘monolingual multilingualism’ is rooted not only in the policy practices of eighteenth and nineteenth century Europe, but also in its intellectual history. The intellectual origins of this conception could be identified in the political writings of three philosophers who sought to reconcile linguistic diversity with social and political unity and the ethical requirements of a democratic polity: Johann Gottfried Herder, Jean-Jacques Rousseau and John Stuart Mill. Their shared preference for state monolingualism resonates today in recent work on language ethics in contemporary political theory. The paper explores the link between language ethics in intellectual history and its contemporary theorising, and the problematic interpretation of ‘linguistic’ that guides it.