Aether (classical element)
Tattva – Mahābhūta
Wŭ Xíng (五行)
Mythological origins 
The word αἰθήρ (aithēr) in Homeric Greek means "pure, fresh air" or "clear sky", imagined in Greek mythology to be the pure essence where the gods lived and which they breathed, analogous to the air breathed by mortals (also personified as a deity, Aether, the son of Erebus and Nyx). It is related to αἴθω "to incinerate", also intransitive "to burn, to shine" (related is the name Aithiopes (Ethiopians)), meaning "people with a burnt (black) visage". See also Empyrean.
Fifth element 
In Plato's Timaeus (St-55c) Plato described aether as "that which God used in the delineation of the universe." Aristotle (Plato's student at the Akademia) included aether in the system of the classical elements of Ionian philosophy as the "fifth element" (the quintessence), on the principle that the four terrestrial elements were subject to change and moved naturally in straight lines while no change had been observed in the celestial regions and the heavenly bodies moved in circles. In Aristotle's system aether had no qualities (was neither hot, cold, wet, or dry), was incapable of change (with the exception of change of place), and by its nature moved in circles, and had no contrary, or unnatural, motion. Medieval scholastic philosophers granted aether changes of density, in which the bodies of the planets were considered to be more dense than the medium which filled the rest of the universe. Robert Fludd stated that the aether was of the character that it was "subtler than light". Fludd cites the 3rd century view of Plotinus, concerning the aether as penetrative and non-material. See also Arche.
The quintessence, as used in alchemy 
The quintessence, or fifth element, was a term used by medieval alchemists for a substance similar or identical to that thought to make up the heavenly bodies. It was proposed that a little of the quintessence was present in things on earth, meaning that things on earth could be affected by what happened in the heavens. This theory was developed in the text “The testament of Lullius”, attributed to Raymond Lull and written in the early 14th century. Alchemy then dealt with the isolation and use of this fifth element.
The idea spread with rapidity through Europe and was popular with later alchemists, especially of the medical sort. This can be seen in “The book of Quintessence”, a 15th century English translation of a continental text. In it, the quintessence is used as a medicine for man’s illnesses, and instructions are given for making it from seven times distilled alcohol. The term has over the years become synonymous with elixirs, medicine or the philosopher’s stone itself.
With the 18th century physics developments some physical models known as "aether theories" made use of a similar concept as an explanation for the propagation of electromagnetic or gravitational forces. However, the early modern aether had little in common with the aether of classical elements from which the name was borrowed. These aether theories are considered to be scientifically obsolete, as the development of special relativity showed that Maxwell's equations do not require the aether for the transmission of these forces. However, Einstein himself 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 an aether in an attempt to address perceived deficiencies in a current physical model. One proposed model of dark energy has been named "quintessence" by its proponents, in honor of the classical element.
See also 
- "ether". The American Heritage Dictionary of the English Language (4th ed.). Boston: Houghton Mifflin. 2006. ISBN 0618701729.
- Pokorny, Julius (1959). Indogermanisches etymologisches Wörterbuch, s.v. ai-dh-.
- G. E. R. Lloyd ), Aristotle: The Growth and Structure of his Thought, Cambridge: Cambridge Univ. Pr., 1968, pp. 133-139, ISBN 0-521-09456-9.
- E. Grant, Planets, Stars, & Orbs: The Medieval Cosmos, 1200-1687, Cambridge: Cambridge Univ. Pr., 1994, pp. 422-428, ISBN 0-521-56509-X.
- Robert Fludd, "Mosaical Philosophy". London, Humphrey Moseley, 1659. Pg 221.
- The Alchemists, by F. Sherwood Taylor page 95
- The Alchemists, by F. Sherwood Taylor page 95
- The book of Quintessence, Early English Text society original series number 16, edited by F. J. Furnivall
- The dictionary of alchemy, by Mark Haeffner
- Einstein, Albert: "Ether and the Theory of Relativity" (1920), republished in Sidelights on Relativity (Methuen, London, 1922)
- Dirac, Paul: "Is there an Aether?", Nature 168 (1951), p. 906.
- Zlatev, I.; Wang, L.; Steinhardt, P. (1999). "Quintessence, Cosmic Coincidence, and the Cosmological Constant". Physical Review Letters 82 (5): 896–899. arXiv:astro-ph/9807002. Bibcode 1999PhRvL..82..896Z. doi:10.1103/PhysRevLett.82.896.