The rigid peripatetic division of the universe into two different parts—one immutable and made of crystalline spheres, situated between the fixed stars and the moon, the other mutable, under the moon—was also related in the Aristotelian paradigm to the concept of the natural sublunar world formed by the four elements: fire, air, water, and earth. Every element behaves differently, according to its so-called “natural movement”. Light elements, such as fire and air, always tend to go upward, while heavy elements, such as water and earth, move downward following a rectilinear movement. Every element aims, in fact, to reach its own natural place, unless an external force causes a movement opposite to its nature—a “violent movement” (e.g., a stone thrown into the air). In both cases, Aristotelian theory considers movement as a quality common to all natural things because of the elements which compose them. By contrast, the heavens, since perfect and composed by a single element (aether), are immutable and travel in constant circular motion around an immovable center represented by Earth. An important corollary to the theory of natural places was the finite character of the world, because it implied the existence of a unique center of the world (the Earth), while evidently it would not be possible for an infinite universe to have a center.