Left to itself, a thing exhibits "natural motion", but can—according to Aristotelian metaphysics—exhibit "enforced motion" imparted by an efficient cause. The form of plants endows plants with the processes nutrition and reproduction, the form of animals adds locomotion, and the form of humankind adds reason atop these. A rock normally exhibits "natural motion"—explained by the rock's material cause of being composed of the element earth—but a living thing can lift the rock, an "enforced motion" diverting the rock from its natural place and natural motion. As a further kind of explanation, Aristotle identified the final cause, specifying a purpose or criterion of completion in light of which something should be understood.