Appetitum naturalem. It is not easy to kill a willow tree, nor to break up an atom of water. Unstable compounds are easily broken up; but the more perfect and stable, with difficulty. This resistance offered to destruction is the 'physical nisus,' often spoken of by the schoolmen under the name of appetitus naturalis. The will is appetitus rationalis; and the region of soul in which passion dwells is a appetitus sensibilis.

Of God and His Creatures: 1.72