  Significantly, the third law thus distinguishes between two kinds of  cases that Descartes believes to be importantly different. The first  kind of case occurs when a moving body with a given power for  proceeding collides with another body that has a greater power for  resisting. Descartes maintains that in such situations the moving  body’s direction or determination is altered, but that nonetheless  the quantity of motion for each of the colliding bodies remains the  same. The second kind of case occurs when a moving body with a given  power for proceeding collides with another body that has a lesser  power for resisting. Descartes maintains that in such situations, the moving body’s determination remains the same, and that the moving  body carries the resisting body along with it in such a way that  their shared total quantity of motion remains the same. The seven  rules further spell out the implications Descartes takes his laws of  motion to have for more specific cases of impact. 