 Boyle was impatient with the Cartesian suggestion that we might be able to alter the direction though not the quantity of motion, not for Leibniz’s reason that the notion of quantity of motion required a confusion between momentum and kinetic  energy,[50]  but for the straightforward reason that interfering with the directed velocity required as mysterious an interaction as altering the ‘quantity of motion’ would. He was aware of the Cartesian claim “that the rational Soul doth [not] give any motion to the parts of the Body, but only guide or regulate that which she finds in them already” (BW, 9:379, Birch 1772, IV:416), but that, he felt, did not really solve the problem, for that interaction was as mysterious as an energy introducing one: “I do as little conceive how the motions of the Conarion can work upon an Immaterial soul, as how any other part of the Body can do it. Nor do I conceive how an Immaterial Soul can work upon the Conarion its self, more then it can upon any other part of the Body” (BP 1:128 BOA §3.5.15, p 253), and he notes that it will not “suffice to object, that the human will does, in these cases, not produce any new motion, but only determine[s] the motions of the spirits, and by their means of the locomotive organs. For to put a check, at pleasure, to the motion of a body, that does already actually move in one line, and determine its motion to continue in another, that is perhaps differing from it, or even opposite to it; to do this, I say, without opposing to the moving body, some other body, which, by its resistance and situation may change its former course, is not a mechanical operation” (BW, 12:480, Birch 1772, VI:756). A change of direction, just as much as a change in the ‘quantity of motion,’ is in fact as mysterious and inexplicable, if done by incorporeal means, as the introduction of energy into the system would be.