  In addition to maintaining that the quantity of motion in the world  is conserved, Descartes held that material bodies are governed by  three laws of motion and seven rules of impact (AT VIIIA 62–71/CSM   1:240–245).[22]  The first two laws in the Principles concern the movements  of bodies in isolation, and taken together constitute Descartes’s  formulation of the principle of inertia. The third law in the  Principles regulates the behavior of bodies colliding under  idealized conditions: 