The word aieńp (aither) in Homeric Greek means "pure, fresh air" or "clear sky". In Greek mythology, it was thought to be the pure essence that the gods breathed, filling the space where they lived, analogous to the air breathed by mortals. Medieval concept of the cosmos is based on five elements. The innermost spheres are the terrestrial spheres, while the outer are made of aether and contain the celestial bodies. Plato mentions that "there is the most translucent kind which is called aether but otherwise he adopted the classical system of four elements. In his Book "On the Heavens" Aristotle introduced a new element to the system of the classical elements of lonian philosophy (earth, water, fire, and air or dry, wet, hot, and cold). He noted that the four terrestrial classical elements were subject to change and naturally moved linearly. The new element however, located in the celestial regions and heavenly bodies, moved circularly and had none of the qualities the terrestrial classical elements had. It was neither hot nor cold, neither wet nor dry. With this addition the system of elements was extended to five and later commentators started referring to the new one as the fifth element called it aether. With the 18th century physics developments, physical models known as "aether theories" made use of a similar concept for the explanation of the propagation of electromagnetic and gravitational forces. As early as the 1670s, Newton used the idea of aether to help match observations to strict mechanical rules of his physics. These aether theories are considered to be scientifically obsolete, as the special relativity and modified version of Maxwell equations do not require the aether for the transmission of these forces. However, Einstein himself - as well as J.C. Maxwell in his original work - noted that his own model which replaced these theories could itself be thought of as an aether, as it implied that the empty space between objects had its own physical properties. Despite the early modern aether models being superseded by general relativity, occasionally some physicists have attempted to reintroduce the concept of aether in an attempt to address perceived deficiencies in current physical models. One proposed model of dark energy has been named "quintessence" by its proponents, in honor of the classical element. This idea relates to the hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has also been called a fifth fundamental force
