For Greek Cosmology,
The concept of aether (or ether) is the indispensable bridge between the divine, eternal heavens and the changing, physical life of humans on Earth.
Aristotle's Definition: The Unchanging Fifth Element (De Caelo)
In his fundamental cosmological treatise, On the Heavens (De Caelo), Aristotle formally introduced aether (which later alchemists and medieval scholars termed the quinta essentia or "quintessence") as a physical element completely distinct from the four earthly elements of earth, water, air, and fire.
While the four sublunary elements only move in straight lines (radially, either falling downward due to weight or rising upward due to lightness) and are subject to constant decay, generation, and death, the aether of the upper heavens is completely incorruptible.
How the Aether Affects Human Life: The Principle of Continuity
How does this perfect sky-substance affect us down here on Earth? Aristotle bridges the gap between the eternal aether and mortal human life in the opening of his Meteorology (Book I, Chapter 2, 339a).
This is the central quote that grounds the entire astrological and cosmological doctrine of celestial influence:
Because the sublunary world (the realm of humans, weather, and the four elements) is physically and energetically contiguous to the heavenly spheres, the circular revolutions of the aether act as the ultimate "efficient cause" of all terrestrial change.
Aristotle argued that the complex, overlapping circular motions of the planets in the sky "step down" their eternal, formatting power through the atmosphere. This celestial movement stirs up the sublunary elements, causing the seasons to turn, the weather to fluctuate, and biological organisms to undergo birth, growth, and decay