Following World War II, intellectual elites in many countries looked to systematic modernization to provide a solution to problems as different as flawed democracy, world hunger, and household chores. In Occupied Japan, dozens of new journals and study groups celebrated science and democracy in a -chorus that observers compared to the modernizing slogan "Civilization and Enlightenment" of the 1870s. The "Institute for the Science of Thought" (Shiso no kagaku kenkyukai), the subject of my dissertation, was one of the most influential associations engaged in rethinking modernity in the years after fascism and defeat. It was founded in Tokyo in January 1946 by an interdisciplinary group of seven young scholars - ranging from nuclear physicists to economists to philosophers. More than a scholarly forum, Science of Thought self-consciously proclaimed a new popular movement that sought to uproot anti-democratic patterns of thought and promote new ways of thinking informed by scientific rationality. The study group and its bimonthly journal represented one of a wide spectrum of efforts by Japanese intellectuals to seize the opportunities created in the immediate postwar years by a democratizing American-led occupation. My project takes Science of Thought as a lens through which to investigate both the radical potential and the historical limits of postwar scientific modernism. Why optimism toward science immediately after the atomic bombs and the Holocaust? What is the legacy of postwar enthusiasm toward science? To what extent did "Science of Thought's vision of scientific modernity actually shape Japan's postwar democracy? In answering these questions through a close study of Science of Thought from its inception in 1946 until its post-Cold War dissolution five decades later, my project explores a phenomenon in the twentieth-century history of Japan, and indeed of the world, that continues to resonate today.