DISCOURSE IV.

ON THE NATURE AND PROPERTIES OF THE AETHER.

Since in the preceding discourse we dealt with the sublunary elements and their affections, as much as seemed sufficient for our purpose, it remains now that we discuss a few things concerning the Aether, or celestial body. However, we define the Aether not as Aristotle did—as a fifth essence, differing in its whole kind from the remaining elements—but as the more recent physicists do: as a most subtle, luminous, and inherently transparent body that fills the heavens.

Definition of Aether.

The nature of this Aether, although not immediately perceived by our senses, is known more than sufficiently from the effects and motions of celestial bodies. For it is, as has been said, a body that is exceedingly thin and fluid, which penetrates not only the spaces between the planets but the celestial bodies themselves, acting as a kind of medium.

Properties of Aether.

Among its principal properties, these are numbered:
1. Extreme subtleness, by which it can penetrate all bodies.
2. Luminosity, by which it transmits light from the stars and planets to us.
3. Transparency, by which it does not hinder luminous rays but promotes them.
4. Incorruptibility, by which it remains immune from the mutations to which sublunary elements are subject.

On the motion of Aether.

As for its motion, the Aether does not rest, but is agitated by a perpetual motion—whether circular or of some other kind—by which the stars are carried in their orbits. It is the common opinion of philosophers that it receives this motion not from itself, but from a first cause, namely God, or from moving intelligences, if one wishes to admit them.