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 amongst Muslims due to the wide reception and assimilation of their monumental encyclopaedia, the Rasa''il Ikhwan al-Safa'' (Epistles of the Brethren of Purity). The present volume is the fourth of this definitive series consisting of the very first critical edition of the Rasa''il in Arabic, complete with the first fully annotated English translation. Prepared by James Montgomery and Ignacio Sánchez, Epistle 4: ''On Geography'' contains a description of the Earth and its climes of great importance to understand both the Islamic geographical tradition and the thought of the Ikhwan al-Safa''.This epistle, which was one of the few geographical works translated into Latin in Medieval Europe, clearly shows the acquaintance of its authors with the Greek Prolemaic tradition, and allows us to trace the main sources used in its composition, namely al-Farghani and al-Khwarizmi. But it is also a propaedeutic work aimed at providing the intellectual tools needed to decipher the signs of God''s Creation, and at understanding the phenomena of the sub-lunar world in the light of the cosmological order. In this regard, ''On Geography'' epitomises many of the central themes in the conceptual universe of the Ikhwan al-Safa'', such as the conception of science as a path towards the superior knowledge of God, and the relationship between microcosmos and macrocosmos as part of the divine design.