24 It was a precise organization, resembling the organization of dangerous secret societies today. In any case, the Brethren of Purity were a secret society working to undermine the Abbasid state in secret, and they had Shi'ite leanings that appear in the folds of the book. They had religious origins and beliefs for this, and they stipulated many precise conditions for joining the membership, and they were arranged encyclopedically. The number of their epistles is fifty-two, dealing with research in mathematics, astronomy, geography, music, ethics, and philosophy. The last epistle among them is considered a summary of these epistles and was written in a refined style, indicating the sophistication of the language of that era. It was adapted to express Arab thought. Al-Ghazali was influenced by it to some extent in his book "Ihya Ulum al-Din". It is mentioned that Al-Ma'arri used to attend the circles of these scholars when he was in Baghdad on Fridays. Abu Hayyan al-Tawhidi mentioned the names of their authors, including Abu Sulayman al-Busti, al-Maqdisi, Abu al-Hasan Ali al-Zanjani, Zayd ibn Rifaa, and al-Qunawi. Al-Tawhidi was the sole source that mentioned their names in his book "Al-Imtaa' wa al-Mu'anasa," which led some to believe that he was a secret member, concealing himself out of taqiyya. The epistles of the Brethren of Purity had varying influences, strong and weak, on scholars in the East and West.