Everybody perceives that the extinction and death of a movement is immobility, when the migration of some body from one place to another ceases and the body remains in the same position in the world space. Nature also shows that weakening and softening of a movement, which is called slowing down [deceleration], can only occur by some mixture and involvement of immobility so that the movement in which more immobility is involved is usually considered as slower. This being established, it seems likely enough and plausible that these causes which could produce immobility and stability in a body can extinguish and stop the movement of this body. Seemingly, this can most properly result from bodies provided with absolute immobility. They should annihilate completely the impetus of a body striking another. However, this is obviously not true. A collision with a body absolutely stable and firm results, not in immobility, but in a movement of resilience carried out with precisely as much impetus as there was velocity in the collision, since the movement of incidence and of recoil is generated completely by the same motive force which changes only its direction and route. Consequently, the immobility and firmness of an obstacle, not only does not annihilate the movement of the striking body, but does not weaken it at all since it keeps the same velocity. Therefore, the death and annihilation of a movement cannot result from the immobility of the opposite body.

It must be seen whether a movement of a body can be annihilated and weakened by the movement of another body and by what kind of movement.

The relation between the movements of two bodies can vary in three ways only. Either both travel on the same straight line in the same direction or in opposite directions, or the movements intersect transversely. And, to start from the end, the transverse movement of a body relative to the movement of another body colliding with the first obviously is not different from immobility since it does neither enhance nor harm the movement of incidence. Indeed it does not impede nor slow it down more than a body absolutely immobile. Consequently, the arriving body rebounds with the same violence from the moving body as it would do from an immobile body. Therefore, it is clear that the impetus of the arriving body is neither weakened nor extinguished by the transverse movement of the obstacle. As far as a movement in the same direction is concerned, unquestionably the body in front escaping the blow of the pursuing body travels either at a velocity smaller than, or equal to, or greater than, that of the pursuing body. If the body in front escapes at a velocity equal to that of the pursuing body, it is clear that the impetus of the body behind and pursuing is in no way weakened since no percussion nor resistance nor repulsion can be imagined in such instance. If the body in front escapes at a greater velocity than that of the pursuing body, even less contact can occur and the bodies part from each other more and more. Therefore, the impetus of the following body can in no way be weakened or extinguished. Finally, if the movement of the escaping body is slower than the impetus of the following body, then certainly the velocity of the pursuing body is slowed down. But it is not a true and proper slowing down since one part of the motive virtue is transmitted and distributed in the body mass of the body in front. Then the same force and velocity remain but they are expanded and transmitted to the body in front. As a result, the body in front escapes at a greater velocity which is increased by as much as the velocity of the pursuing body was decreased. But, as I said, this diminution and slowing down are not those of which we speak. Indeed we deal with the true and actual diminution and extinction of the movement and motive virtue, with that which does not migrate from one subject to another but is truly extinguished and stops being.