On Nicomachus's Introduction to Arithmetic

...for the monad always approaches it dyadically, whether unmixed or combined, whether pure or with any homogeneous thing whatever. The odd is from the opposite; it is measured specifically by the monad when it is odd-numbered, and accidentally by the dyad, though not by itself, but together with the monad. The monad, however, has a unique quality apart from all odd numbers, as their formative principle, namely that it is not divided into unequal parts; and the dyad has a unique quality apart from even numbers, namely that it is divided only into equal parts. For this reason, the Pythagoreans named the monad Atropos the inflexible/one of the Fates and Apollo, and other such names, and the dyad Isis and Artemis by analogy. Because the monad is naturally indivisible, it will appear as a limit and definition for both sides: for magnitude, so that the division to infinity may begin from it as a whole; and for quantity, so that the increase to infinity may be extended in the same way as the monad. And as from a whole, a half arises, then a third, then a fourth, then a fifth, and so on, always larger and more distinct parts, proceeding contrary to the increase of the names; just as from the monad, the dyad arises, then the triad, then the tetrad, and so on, a progression to infinity, the increase and the generation of varied counter-naming arise according to the names, with the monad subsisting...