primum mobile

noun3

1. •
   (historical) The outermost celestial sphere of the heavens in Ptolemaic astronomy, which was believed to cause all the inner spheres to rotate.

   • Dante [Alighieri] now mounts up from the Heaven of the fixed stars to the Primum Mobile, or Crystalline Heaven. — 1867
2. •
   The prime mover or first cause (“an initial cause from which all other causes and effects follow”).
3. •
   The person or thing that is the main impetus for some action; a driving force.

   • The standard metaphor for the role of the grand jury in early modern law was the primum mobile, the wheel that allowed all other wheels to begin their turning. — 1987