Only preserved in a single manuscript in Tehran, this remarkable twelfth-century Qur'anic commentary by Muhammad b. 'Abd al-Karim al- Shahrastani marks the achievement of a lifelong, arduous quest for knowledge. Shahrastani began writing Mafatih al-asrar or Keys to the Arcana towards the end of his life and the work reflects the brilliant radicalism of his more private religious views. The introduction and opening chapter of this virtually unknown work is presented here in a bilingual edition, which also includes an introduction and contextual notes by Toby Mayer.In Keys to the Arcana, Shahrastani breaks down the text of the Qur'an and analyses it from a linguistic point of view, with reference to the history of Qur'anic interpretation. The author's ultimate aim is to use an elaborate set of complimentary concepts - the 'keys' of the work's title - to unearth the esoteric meanings of the Qur'anic verses, which he calls the 'arcana' of the verses (asrar al-ayat). A historian of religious and philosophical doctrines, Shahrastani has generally been considered to be a spokesman for the Sunni religious establishment under the Seljuqs. The complimentary concepts in question, however, appear to derive from the Isma'ili Shi'i intellectual tradition, indicating that the author may have been secretly involved in the Isma'ili movement. Shahrastani 's unusually esoteric and highly systematic exegesis of the Qur'an provides a vivid picture of the mature state of scriptural commentary in the twelfth-century CE. Dr Mayer's meticulous translation of Shahrastani 's Introduction and Commentary on Surat al-Fatiha, supplemented by the Arabic text, allows the reader and scholar access to this intriguing Muslim intellectual work for the first time.