We demonstrated that not all bodies are moved by a distinct and separate motor. But finally we must consider the moving subject which moves by itself. It seems incredible of course that his motive impetus could not be impressed by God the creator into some bodies when he created them. A projected body, colliding with an immobile and movable hanging body, was allowed to be able to impress into this a degree of impetus thanks to which the body can be displaced afterwards. Consequently, although in the nature of things bodies are supposed to be absolutely inert and immobile, some spirituous and lively bodies must also be admitted. They can move by themselves by a natural virtue transmitted to them by God. This movement of spirits seems to be uniform. Their motive force produces movement as far as there is movement. Consequently, the end and desire of nature being complied with, i.e. a movement resulting at the velocity which its natural force requires, the effort towards further and more impetuous movement necessarily stops. Therefore, the movement will not be accelerated and, if this movement is slowed down by some obstacle or ceases, then the motive force must not be considered as weakened or annihilated since this motive force is never inactive nor dead but always impels the obstacle with the same effort. In a boat carried by the stream of a river, a movement of a seaman from the bows to the stern at the same velocity as that of the boat, is not annihilated by the opposite movement of the boat although the seaman does not achieve the effect of his movement, i.e. a displacement in the space of the world. The movement of the seaman indeed is always carried out and effective but it is in some way hidden as a result of its combination with the opposite movement of the boat. If then the obstacle constituted by the opposite movement of the boat is removed, the movement of the seaman becomes conspicuous and observable. There is thus in the first instance a lack of displacement but not of movement. It is the same when a body which moves by itself meets a firm obstacle. Then no inert and dead contact occurs but continuous struggle and resistance. The forces thus balanced carry out a tonic action and motion. After the obstacle is removed, the body manifests and displays by moving freely in the space of the world that force which it exerted in this struggle in equilibrium.