The Ikhwan al-Safa' (Brethren of Purity), the anonymous adepts of a tenth-century esoteric fraternity based in Basra and Baghdad, hold an eminent position in the history of science and philosophy in Islam due to the wide reception and assimilation of their monumental encyclopaedia, the Rasa 'il Ikhwan al-Safa' (Epistles of the Brethren of Purity). This compendium contains fifty-two epistles offering synoptic accounts of the classical sciences and philosophies of the age; divided into four classificatory parts, it treats themes in mathematics, logic, natural philosophy, psychology, metaphysics, and theology, in addition to didactic fables. Epistles 43-45 succeed the extended description of religions and creeds that opens the final section of the corpus, on the theological sciences. Epistle 43 explains briefly the need for purifying one's soul by performing virtuous acts, after which one can follow the "straight path" to God. The extremely diverse Epistle 44 is no dry exercise in abstruse theology; rather, it is characterized by the most delightful anecdotes, designed to inform the reader of a deeper truth, that of the hereafter following the soul's separation from the body at death. Alongside references to many of the Prophets encountered in the Qur'an, this Epistle shows a familiarity with other religions such as Christianity, Judaism, and Buddhism. Epistle 45 focuses on companionship and friendship, resonating strongly with the word "Brethren" by which the authors distinguish themselves. The volume overall is united in its underlying themes of the immortality of the soul and the profound need for mutual cooperation, informed in parts by the general Neoplatonism of the entire corpus, as well as by Aristotelian and Platonic motifs.