Simple bodies are terminated according to species.
_Demonstration._—For let the magnitude A be a simple body. Since,
therefore, a simple body is moved with a simple motion, A will be moved
with a simple motion. And if it is moved in a circle, it will have one
nature and one form. But if it is moved according to any one of the
motions in a right line, if it is moved from the middle only, it will
be fire, but if only to the middle, earth. But, if it is light with
respect to one thing, and heavy with respect to another, it will be some
one of the middle elements. The species therefore of simple bodies are
terminated.