Aristotle argued that the natural state of an object was to be at rest. It would only move it force was applied to it by an externality. Objects that were not in motion were in this state because force had ceased to be applied to them. From his physics, he extrapolated several important ideas. One of these ideas was that nature itself was change, and motion. Another was that the heavens were constantly in motion. From these conclusions, and many others, he formed the idea of a "prime mover," something which was eternal (and constantly at rest) yet something which was capable of exerting the force which first put objects in motion. The heavens were put in motion by the prime mover, while earthly objects were put in motion by contact with other objects.