The Summary of Physics Restored

51The bodies of earth, water and air, sensibly distinguished in their own spheres, are different from the elements that nature uses in generation and that constitute the bodies of compounds. In mixture these are imperceptible through their fineness and hidden from the senses until they come together into dense matter and a body:

51.2For acknowledge that all things consist of imperceptible principles.

51.3Those that constitute the universe's lower globe, however, are excluded from the law of perfect generation as too dense, crude, impure and undigested. They are images and likenesses of the elements rather than true elements.