In 1906 Martin Schreiner, who had been diagnosed with mental illness four years previously, wrote from the Sanatorium Berolinum in Berlin-Lankwitz to the librarian of the "Lehranstalt für die Wissenschaft des Judentums" in Berlin, asking to send him some books, in the hope of being released soon. This letter and some short letters dating from 1920 and 1922 constitute Schreiner's last written testimonies. His mental illness painfully and suddenly ended the short but productive career of a versatile scholar who was one of the most important exponents of the "Wissenschaft des Judentums" and who simultaneously engaged in the study of Islam. Sabine Schmidtke reconstructs Schreiner's scholarly biography from his student days to his active period in Berlin, where the manuscript holdings of the Royal Library opened up entirely new perspectives for him. The author focuses on his pioneering scholarship particularly in the field of Islamic intellectual history, where his main contributions dealt with the Muʿtazila, Ibn Taymiyya and his circle, the mystical tradition of Ibn ʿArabī, and interreligious polemics.