This article includes a translation of the part of the treatise "Epistle on Sufism" by one of the most famous sufy theoretics of the Xth century al-Qushchayi (986—1072), as well as the commentary to his work. For the translation was selected the first part of the treatise, that hasn't been yet translated to Russian and which describes the author's main aims and goals. The article emphasizes on the importance of this work in the history of sufism.