The "Rasa’il Ikhwan al-Safa’" ("Epistles of the Brethren of Purity") consist of fifty-two treatises in mathematics, natural sciences, psychology (psychical sciences) and theology. The first part, which is on mathematics, groups fourteen epistles that include treatises in arithmetic, geometry, astronomy, geography, and music, along with tracts in elementary logic, inclusive of: the "Isagoge", the "Categories", "De Interpretatione", the "Prior Analytics" and the "Posterior" "Analytics". The second part, which is on natural sciences, gathers seventeen epistles on matter and form, generation and corruption, metallurgy, meteorology, a study of the essence of nature, the classes of plants and animals, including a fable. The third part, which is on psychology, comprises ten epistles on the psychical and intellective sciences, dealing with the nature of the intellect and the intelligible, the symbolism of temporal cycles, the mystical essence of love, resurrection, causes and effects, definitions and descriptions. The fourth part deals with theology in eleven epistles, investigating the varieties of religious sects, the virtue of the companionship of the Brethren of Purity, the properties of genuine belief, the nature of the Divine Law, the species of politics, and the essence of magic.