With the Weimar Republic, an era of engaged reason came to an end. Thinkers from various disciplines fought against the rising political irrationalism with the tools of scientific sobriety and rationality. The fact that this is now mostly perceived as purely descriptive logic, beyond its life-world and normative aspects, is a consequence of a division of reason that began with the rise of National Socialism and its intellectual forerunners, and which continues to occupy science to this day. This book describes the historical and scientific context in which scientists like Albert Einstein, Moritz Schlick, and Ludwik Fleck were engaged, and uncovers buried potentials for a historical understanding of rationality that encompasses both science and its life-world environment.