Selection from St. Thomas Aquinas, On the Principles of Nature
Trans. R. A. Kocourek, cc. 8-9.
8. In order that there be generation three things are required: being in potency which is matter, non-existence in act which is privation, and that through which something comes to be in act which is form. For example when a statue made from bronze the bronze which is in potency to the form of the statue is the matter; the shapeless or undisposed something is the privation; and the shape because of which is called a statue is the form.. . .
9. Therefore there are three principles of nature: matter, form and privation. One of these, form, is that by reason of which generation takes place; the other two are found on the part of that from which there is generation. Hence matter and privation are the same in subject but they differ in definition, because bronze and what is shapeless are the same before the advent of the form; but for one reason it is called bronze and for another reason it is called shapeless.