DISCOURSE IV.

ON THE NATURE AND PROPERTIES OF THE AETHER.

Although many things could be disputed in this treatise regarding the nature of the Aether—as it is a most subtle substance and far removed from our senses—it is nevertheless permitted to gather some more certain facts from the principles of natural philosophy that have become known to us. For the Aether is to be considered not as a vacuum, but as a fluid body, diffused through all the spaces of the world.

Aristotle, On the Heavens, book 1.

Ancient philosophers, especially Aristotle, called the Aether the "fifth essence," distinct from the four elements and suited for circular motion. We, however, relying on more recent physics, consider the Aether to be a subtle matter that carries light and heat through immense spaces.

PROPOSITION I.

The Aether is a most fluid body, and one that penetrates all bodies.

The reason for this is that the Aether, being a most thin matter, cannot be excluded by any pores, however narrow. From this, it is clear why celestial bodies, moved through the Aether, suffer no sensible resistance.