According to Aristotle, what tendency of moving objects governed their motions?

Aristotle interpreted two kinds of motion in universe, the vertical motion of earth objects and the circular motion of celestial objects. He explained it basically as its NATURE, compared like water is below, above is air. Those days they lacked the concepts of inertia (which never allows to stop or start a motion, which means every motion is caused) and force (which mainly includes gravitational force).