The renowned psychologist Kurt Lewin (1890-1947) is considered the true founder of experimental social psychology, group dynamics, and action research. His work in these areas emerged after Lewin's emigration to the USA. However, even during his time in Germany, when he was active at the Berlin Psychological Institute alongside Wolfgang Köhler and Max Wertheimer, Lewin proved to be a creative and innovative researcher. These early works, however, remain largely unknown. In this book, the author presents a comprehensive overview of this particular phase of Lewin's work. For the first time, the outstanding role of psychologist Kurt Lewin as a philosopher of the Weimar era is documented in a scientific historical context. This also allows for a deeper exploration of the theoretical foundations of Lewin's empirical-practical research, including his work during his time in America.