Aristotle distinguished between two sorts of motion: natural and unnatural. Natural motions were those induced by the elemental constituents of things to seek their natural places—the earth at the center of the cosmos, fire under the periphery, water and air in their intermediate locations. Anything not in its natural place (i.e., not at that point in the stratification of things appropriate to its elemental composition) has an internal inclination to reach its natural place that will be exercised so long as nothing impedes it. Moreover, the speed of any body in natural motion (i.e., approaching its natural place) is a function of its heaviness in the case of downward motion, lightness in the case of upward, and an inverse function of the resistance of the medium through which it moves.

Not all motions, however, are natural; heavy objects can be hurled upwards, buoyant ones forcibly...