Things which are naturally moved in a circle, neither participate of
gravity nor levity.
_Demonstration._—For if AB is either heavy or light, it is either
naturally moved to the middle, or from the middle: for, from the
definitions, that is heavy which is moved to the middle, and that is
light which is moved from the middle. But that which is moved either
from or to the middle, is the same with some one of the things moved in
a right line. AB, therefore, is the same with something moved in a right
line, though naturally moved in a circle, which is impossible.