THEOREM 7.
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.