out theories of neurology and sensory perception and the effects of astrology on portions of the body. Embryology. Which planets control which month of pregnancy. In spirit, a continuation of the ninth, which begins: "Know, O brother, that the knowledge of one's own self is the key to every science and this is threefold; first, man ought to be acquainted with the component part and economy of his own body, and with all those qualities which are independent of the influences of the soul; secondly, he ought to study the soul and its qualities independent of the body, and thirdly he ought to understand their joint action." The Brethren further go on to lay astrological correspondences with bodily parts and orifices. "On the modalities of birth of the particular souls in the natural human bodily systems." The Institute of Ismaili Studies: The Classification of the Sciences according to the Rasail Ikhwan al-Safa. Davidson offers a clearer gloss: "...shows how the partial soul grows in the human body, and how it may thus, before or after death, become an angel." van Reijn reverses this and the previous one, holding that number 25 is on the embryo and 26 is on "Man as Microcosm". "On the extent of the powers of the human mind to penetrate into the mysteries of the universe" and the mysteries of the Creator. On life and death, and how a rational soul is embodied, and why death is not to be feared, since it allows the good to reach Paradise. (van Reijn likewise reverses this and the preceding one). Pleasure and pain, in mortal and immortal life. "The Diversity of Languages" On the purposes and functions of language, and also on the cause of differing languages. Psychological and Rational Sciences A Pythagorean explanation of creation, a defense that the universe was indeed created in a way deeply concerned with numbers at some point, and that the decimal system- then a still fairly recent innovative introduction from the Hindus - described the universe. "On the origins of the logos (i.e. intellect considered as a substance and not as a faculty)." Eric van Reijn glosses this epistle as concerning the "Spiritual Creation (according to the Sincere Brethren)." "Man as Macrocosm." Further discussion of a macrocosmic correlation with a microcosm of man's body. "On intellect (as a faculty of the mind), and the object of intellect." "On the revolutions and