Background: There is a common sense in U.S. education research that the current political moment is one defined by increasing polarization, accusations of educational indoctrination, and concerns of duped minds. To address fears of indoctrination, education research and major educational organizations have renewed calls for civic reasoning, deliberation, and scientific inquiry to foster a more rational, responsible citizenry. Focus of Study: This article examines 'the discriminating mind' as a set of principles and practices relating to 'reason' that distinguish between scientific (versus prejudicial) thinking and democratic (versus authoritarian) attitudes. By investigating these principles and practices, the authors illustrate how 'the discriminating mind' became both a psychological ideal and pedagogical imperative. The analysis traces the emergence of this idealized imperative to earlier moments when U.S. democracy appeared likewise threatened by political divides. Across these periods, appeals to science offered a means to 'cool' the 'hot' passions of an 'unreasoning' public. Research Design: The paper historicizes the discriminating mind within education research, examining early 20th-century efforts to reduce race prejudice within Americanization reforms and comparing these with mid-century intercultural and intergroup education. It draws from historiographies of 20th-century social science and from primary documents (e.g., educational research journals, psychological reports, curriculum guides), mapping the principles and practices associated with 'reason.' The analysis identifies a paradox in claims that rational discrimination transcends prejudices, because such claims have historically relied upon their own racializing presumptions of who is not yet ready for democracy. Conclusions: Historicizing the discriminating mind makes visible the foundations grounding both calls to ban teaching about racism and proposals to develop rational, anti-racist citizens. Both rely upon principles and practices that divide rational citizens from those bound to emotion, dogmatism, and ideology. In so doing, the imperative to make discriminating minds treats equality as a potential to be appraised rather than an actuality to be affirmed. By historicizing this imperative, the analysis suggests that efforts to appraise individuals' and populations' civic potential are an ineffective strategy to confront authoritarianism.