  Bacon's cosmological system—a result of thought experiments and speculation, but not proven in accordance with the inductive method—presupposes a finite universe, a geocentric plenum, which means that the earth is passive and consists of tangible matter. The remaining universe is composed of active or pneumatic matter. Whereas the interior and tangible matter of the earth is covered by a crust which separates it from the pneumatic heaven, the zone between earth and the “middle region of the air” allows a mixture of pneumatic and tangible matter, which is the origin of organic and non-organic phenomena. Bacon speaks here of “attached spirit” (Rees 1986, 418–20), while otherwise he assumes four kinds of free spirit: air and terrestrial fire, which refer to the sublunary realm; ether and sidereal fire, which are relevant to the celestial realm. Ether is explained as the medium in which planets move around the central earth. Air and ether, as well as watery non-inflammable bodies, belong to Bacon's first group of substances or to the Mercury Quaternion.