The Epistles of the Brethren of Purity (9th-10th cent.) constitute one of the most enigmatic works in the history of Islamic philosophy. This book presents an in-depth study of the Epistles and its two summaries. Mattila argues that the Ikhwānian corpus evolved from a mainly philosophical encyclopedia of the Epistles to the largely religious treatises of its summaries. Based on philological analysis, the book's first part shows the gradual transformation of the contents of the Epistles. The second part demonstrates that the Comprehensive Epistle was not composed by the same authors but instead represents an Ismāʿīlī adaptation of the encyclopedia.