Aristotle proposed a fifth element, aether, in addition to the four proposed earlier by [[Empedocles]].
*[[Earth (classical element)|Earth]], which is cold and dry; this corresponds to the modern idea of a solid.
*[[Water (classical element)|Water]], which is cold and wet; this corresponds to the modern idea of a liquid.
*[[Air (classical element)|Air]], which is hot and wet; this corresponds to the modern idea of a gas.
*[[Fire (classical element)|Fire]], which is hot and dry; this corresponds to the modern ideas of [[Plasma (physics)|plasma]] and [[heat]].
*[[Aether (classical element)|Aether]], which is the divine substance that makes up the [[Celestial spheres|heavenly spheres]] and heavenly bodies (stars and planets).
Each of the four earthly elements has its natural place. All that is earthly tends toward the center of the universe, i.e., the center of the Earth. Water tends toward a sphere surrounding the center. Air tends toward a sphere surrounding the water sphere. Fire tends toward the lunar sphere (in which the Moon orbits). When elements are moved out of their natural place, they naturally move back towards it. This is "natural motion"—motion requiring no extrinsic cause. So, for example, in water, earthy bodies sink while air bubbles rise up; in air, rain falls and flame rises. Outside all the other spheres, the heavenly, fifth element, manifested in the stars and planets, moves in the perfection of circles.