ENThe article presents an unknown treatise written by a Vilna-based maskil and medical doctor, Josef Rosensohn (ca. 1774-1849). The newly discovered text, commissioned by a Polish aristocrat and Enlightenment activist Tadeusz Czacki (1765–1813), is the oldest maskilic treatise on Hasidism, written in either late 1804 or 1805, important not only for the history of the Haskalah, but also a significant source on the history of Hasidism and Jewish-Polish cultural relations. The article consists of two parts: (1) extensive introduction presenting the author, circumstances of the treatise creation, and significance of the source; (2) a thoroughly annotated treatise itself. Keywords: Vilnius (Vilna), Lithuania, Poland, Haskalah, Hasidism, Enlightenment, Josef Rosensohn, Tadeusz Czacki, Shneur Zalman of Liady. [From the publication]