Next, we must ask, why does this heavenly fluid move in vortices, like a whirlpool. This claim rests on principle II.33, in which Descartes proves that all motion must be in the form of a continuous circuit. Since all of space is a plenum, in order for A to move to B's spot, B must move to C's spot and C to D's spot and so on. As each tiny body takes the other's spot in the quickly moving heavenly fluid, a vortex, swirling around the center, is formed.
Descartes is then able to use the physics of vortices to explain certain other observable phenomena, particularly the relative motion of the planets. Those planets that are positioned toward the center of the vortex naturally move around the sun the quickest, since in a vortex the middle swirls most rapidly. (Just imagine a whirlpool to understand why this is true).