St Thomas refers to Metaph. VII, viii (now VI vii), apparently to such words as these: kathalou de kai ex hou phusis, kai kath ho phusis, to de gignomenon echei phusis, hoion phuton ê zôon (universally, the source whence a thing proceeds is nature, and the process nature, and the product is natural as a plant or an animal). This means that 'natural,' i.e. 'organic,' beings attain to a certain completeness, proper to themselves, by a development of their own powers they are self-contained and self-sufficient for their own purposes: they draw indeed their supplies from without, but they adapt what they receive to their own purposes by their own activity.
Of God and His Creatures: 2.76