De institutione musica II.2: Quid Pythagoras esse philosophiam constituerit.
Pythagoras was the first of all to call the pursuit of wisdom philosophy, which he took to be the knowledge and discipline of that which is properly and truly said to be. And he held that those things are which neither grow by increase, nor decrease by diminution, nor are changed by any accidents. These he held to be forms, magnitudes, qualities, dispositions, and the rest, which, considered in themselves, are unchangeable, but joined to bodies are altered and, by kinship with changeable matter, are turned by manifold variations.