[102] [cap.21] Now, those who have applied themselves to mathematical studies, fully explain the precedence and pre-eminence to which the number seven is entitled among all existing things, tracing it out with great care and exceeding minuteness and accuracy; for among numbers seven is the virgin number, the nature which has no mother, that which is most nearly related to the unit, the foundation of all numbers; the idea of the planets, just as the unit is of the immovable sphere; for of the unit and the number seven consists the incorporeal heaven, the model of the visible heaven, and the heaven is made up of indivisible and divisible nature.