 Bacon had no explanation for the planetary retrogressions and saw the universe as a finite and geocentric plenum, in which the earth consists of the two forms of matter (tangible and pneumatic). The earth has a tangible inside and is in touch with the surrounding universe, but through an intermediate zone. This zone exists between the earth's crust and the pure pneumatic heavens; it reaches some miles into the crust and some miles into air. In this zone, pneumatic matter mixes with tangible matter, thus producing ‘attached spirits’, which must be distinguished from ‘free spirits’ outside tangible bodies.  Bacon's four kinds of free spirits are relevant for his ‘quaternion theory’: