The Encyclopedia of the Brethren of Purity (Rasā'il Ikhwān al-ṣafā') also variously known as the Epistles of the Brethren of Sincerity, Epistles of the Brethren of Purity and Epistles of the Brethren of Purity and Loyal Friends is an Islamic encyclopedia"The work only professes to be an epitome, an outline; its authors lay claim to no originality, they only summarize what others have thought and discovered.