 There are also several encyclopedic works of the Ismā‘īlī tradition whose authors are unknown. Many of these deal with mystical and Sufi ideas. Among the works that have been influential in the spread of gnostic ideas are treatises like Brethren of Purity (Ikhwān al-ṣafā) and The Mother of Books (Umm al-kitāb) (Daftary 1990; Nasr &amp; Aminrazavi 1999/2001).