Excerpt on Aristotle's theories of motion

From page 59 of Foundations of Modern Science in the Middle Ages, by Edward Grant.

"Aristotle distinguished two kinds of motion: natural and violent (or unnatural), a division that probably originated in gross observation. The division of local motion into natural and violent and the cluster of concepts, arguments, and physical assumptions associated with these two contrary motions formed the core of Aristotle's sublunar physics."