<P> In physics, theories of gravitation postulate mechanisms of interaction governing the movements of bodies with mass . There have been numerous theories of gravitation since ancient times . </P> <P> In the 4th century BC, the Greek philosopher Aristotle believed that there is no effect or motion without a cause . The cause of the downward motion of heavy bodies, such as the element earth, was related to their nature, which caused them to move downward toward the center of the universe, which was their natural place . Conversely, light bodies such as the element fire, move by their nature upward toward the inner surface of the sphere of the Moon . Thus in Aristotle's system heavy bodies are not attracted to the earth by an external force of gravity, but tend toward the center of the universe because of an inner gravitas or heaviness . </P> <P> In Book VII of his De Architectura, the Roman engineer and architect Vitruvius contends that gravity is not dependent on a substance's "weight" but rather on its "nature" (cf . specific gravity). </P>

Who came up with the idea of gravity