In the 4th century BCE, Greek philosopher Aristotle taught that there is no effect or motion without a cause. The cause of the downward natural motion of heavy bodies, such as the element earth and water, was related to their nature (gravity), which caused them to move downward toward the center of the (geocentric) universe. For this reason Aristotle supported a spherical Earth, since "every portion of earth has weight until it reaches the centre, and the jostling of parts greater and smaller would bring about not a waved surface, but rather compression and convergence of part and part until the centre is reached". On the other hand, light bodies such as the element fire and air, were moved by their nature (levity) upward toward the celestial sphere of the Moon (see sublunary sphere). Astronomical objects near the fixed stars are composed of aether, whose natural motion is circular. Beyond them is the prime mover, the final cause of all motion in the cosmos. In his Physics, Aristotle correctly asserted that objects immersed in a medium tend to fall at speeds proportional to their weight and inversely proportional to the density of the medium.