Etymologiae, Book III, chapters 23-27: De numeris musicis to De differentia astronomiae et astrologiae
Chapter XXIII. On musical numbers You seek numbers according to music thus. The extremes being posited, say 6 and twelve, you see by how many monads 6 is exceeded by 12, and it is by 6 monads: you multiply by the square; six times six make 36. You join those first extremes, 6 to 12, together they make 18. You divide thirty-six by eighteen, it makes two. These you join with the lesser sum, that is, six; there will be 8 and it will be the mean between 6 and 12. Wherefore 8 exceeds 6 by two monads, that is, a third of 6, and 8 is exceeded by 12 by four monads, a third portion. By what part, then, it exceeds, by the same it is exceeded. But as this reckoning is in the world from the revolution of the circles, so in the microcosm it avails so far beyond the voice that without its perfection even man, lacking symphonies, does not stand. By the perfection of the same music the metres also consist in arsis and thesis, that is, raising and setting down.
Chapter XXIV. On the name of astronomy Astronomy is the law of the stars, which traverses, with searching reason, the courses of the heavenly bodies and the figures and habits of the stars around themselves and around the earth.
Chapter XXV. On its inventors The Egyptians first invented astronomy. The Chaldeans first taught astrology and the observance of nativity. The author Josephus asserts that Abraham instituted astrology among the Egyptians. The Greeks say that this art was first devised by Atlas, and that therefore he was said to have held up the heavens. Whoever he was, roused by the motion of the heavens and by reason of the mind, through the vicissitudes of the times, through the fixed and defined courses of the stars, through the measured spaces of the intervals, he considered certain dimensions and numbers, and by defining and separating, binding into order, he discovered astrology.
Chapter XXVI. On its institutors In both languages, however, there are volumes of various writers on astronomy, among whom Ptolemy, king of Alexandria, is held chief among the Greeks: he also established canons by which the courses of the stars may be found.
Chapter XXVII. On the difference between astronomy and astrology Between astronomy and astrology there is some difference. For astronomy embraces the revolution of the heavens, the risings, settings and motions of the stars, and for what cause they are so called. Astrology is partly natural, partly superstitious. Natural, when it follows the courses of the sun and moon, or the fixed stations of the stars in their seasons. Superstitious is that which the mathematicians follow, who divine by the stars, and who also set the twelve signs of heaven over the single members of the soul or body, and by the course of the stars try to foretell the nativities and characters of men.