Heronis Alexandrini spiritalium liber

We set out to establish this, as has been said. For through the combination of air, fire, water, and earth—the three elements, or rather four, colliding with one another—various arrangements are produced: some indeed providing the necessary uses for our life, others stirring up a certain terrible admiration.

A circular diagram depicts the four elements (air, fire, water, earth) interacting, symbolizing the foundational forces of the physical world in ancient natural philosophy.

On the Vacuum.

But before we approach the matters that are to be discussed, one must debate the vacuum. For some affirm that there is no vacuum at all in the universe; others, that while there is no vacuum concentrated by nature, it is indeed disseminated in small parts throughout the air, the moisture, the fire, and other bodies; and it is fitting to agree mostly with these. For from those things that appear and fall under the senses, this will be demonstrated to be so in the following. For vessels that appear empty to many are not empty, as they think, but full of air. Air, however, as is agreed by those who treat of nature, consists of small and light bodies, mostly not appearing to us.