DISCOURSE IV.

ON THE NATURE AND PROPERTIES

OF THE AETHER.

Since we have dealt in our previous discourse with the sublunary elements and their affections as they occur in this lower world of ours, it now remains for us to discuss the Aether—that most pure element which is diffused throughout the celestial spaces above the Moon.

The Aether, as defined by philosophers, is a celestial body: luminous, subtle, and perennial. It surrounds all celestial bodies, and the stars swim within it as if in a certain sea. This Aether is not, as some of the ancients dreamed, a "fifth essence" original: "quinta essentia" distinct in its entire kind from the remaining elements, but rather a most pure fire, refined and purged of all earthly dregs.

The Properties of the Aether

1. Subtlety: The Aether is the most subtle of all bodies, to such a degree that it cannot be perceived by any of our senses except through its effects, which are seen in the motion of the stars and in the light it transmits to us.
2. Luminosity: It is luminous in itself and is the source of all light that is diffused throughout this universe.
3. Incorruptibility: Although all things in this lower world are changeable, the Aether, being free from coarse matter, is not subject to corruption.
4. Motion: The motion of the Aether is circular and uniform, by which the stars are carried around in their orbits.

Aristotle, On the Heavens, Book 1.

Aristotle, in his book On the Heavens, distinguishes the Aether from the four elements, asserting it to be a divine nature that possesses neither gravity nor levity, but only circular motion, which is the most perfect. We, however, following more recent astronomers, consider the Aether to be that empty space, or rather that most subtle fluid, which contains the celestial bodies.