[St. Pachomius Library]



In Greek philosophy, the hypothetical fifth element, of which the heavens might be composed. It was supposed to have the property of self-propulsion along a circle, thus explaining why the heavens keep moving around the Earth. It was also associated with the dodecahedron in geometry; the construction of the dodecahedron is the last problem in Euclid's Elements. By the Middle Ages, it had been identified with the alchemical Philosopher's Stone.

In more modern times, the term has lost its connotation of self-movement, and refers only to a substance imagined as filling the vacuum of "empty" space. Because removing the necessity of such an "æther" was a major achievement of special relativity, that term is now rarely used in science, although the concept has re-entered cosmology under a variety of other names, including the venerable synonym "quintessence".

--- Norman Hugh Redington

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