Compendium of All Natural Science

...are those which seem to be bound by no end, such as death. For death is nothing; wherefore it will have no end. Some things, however, have happened in such a way on account of matter, such as monsters. For just as with artifacts, so too with natural things, necessity sometimes depends somewhat upon the matter. Yet the actions of nature are governed chiefly by the end (as we have said). For this reason, things composed of elements do not follow the order of the elements: because it is necessary that they be ruled by the end, not by the matter. For even bones, which are heavier, are placed in the head; and blood, which for the most part is lighter, travels through the lowest part of the body. For nature provided that any composition whatsoever should be ruled by the end rather than by the matter. But there are also some things which borrow their necessity from the form, such as those things which are concluded by mathematical demonstration. Certain things also depend upon the efficient cause: as the illumination of a portico proceeds from the sun providing light.

18. On motion.

Since Nature is truly the principle of motion and rest, we shall not know Nature until we understand what rest and motion are. Therefore, whatever things are contained in the heaven, the earth, and finally the whole world, either remain stable in their parts or slip away. The former are called constant, the latter flowing. Constant things are such as a man or a stone; flowing things are such as time or motion. Motion, moreover, is the flowing and incomplete perfection of the thing which is being moved. Truly, whatever things are moved, either...