Incorruptible celestial bodies

Meaning

II.2.12 calls the celestial source bodies immortal, incorruptible and unfailing, and infinite in powers. These predicates explain why the powers continuously arriving from them can be called infinite when united at earth’s centre. The heavens’ subtle material constitution also explains the subtlety of the penetrating influence: operations follow the qualities of their producing bodies.

The receiving and accumulating subject here is earth, not the celestial bodies themselves. Penetration without a pause would be like water running too quickly over a field to wet more than its surface. At the centre, where the author says there is nothing lower, the influences must stop and gather. The account attributes permanence to the celestial sources; it is neither modern astronomy nor a promise that an earthly medicine grants humans that same permanence.