Ignorabimus! We will never know! With this statement and his talk on the limits of natural knowledge in 1872, Emil du Bois-Reymond stirred up a controversy (the Ignorabimus-Streit), which spread widely beyond German-speaking countries. It concerned the very possibility to set boundaries to knowledge, the development of the sciences, their attainable results, and concept formation. In this volume, the philosophical value of the Ignorabimus controversy is critically examined. The historicalmatter and its theoretical implications are assessed with regard to the mutual relationships between philosophy and the sciences in the 19th century and beyond.