Having to deal with the force and energy of the act of percussion, I will at first as shortly and as clearly as I can explain some notions concerning movement in general: what movement is, what its causes and its principles are, what its effects are, how it is provoked or stopped, and other points of this kind.

Firstly, as far as the definition of movement is concerned, it is known, as we described elsewhere, that a definition is a concept or an idea through which we perceive clearly the nature of the defined object and through which we distinguish and separate this object from anything else. We obtain the definition by selecting a property or a quality of this object which is the best known of all which belong to it. The most obvious characteristic of the bodies which are displaced or moved, and which is absent in all the bodies at rest and immobile is nothing else than their passage or migration from one place to another. A displacement indeed is the successive passage from one place to another in a well-determined time while touching one after the other all the successive parts of the place or of the space travelled through. Therefore, movement (something certainly obvious) needs no ambiguities and profuse description. It seems to be a state of the bodies in which it exists or acts. It resolves into a kind of uninterrupted quantity, non permanent but successive, since a movement has such relation with the time during which it occurs that there is nothing else in the movement itself than the indivisible instant in time during which it occurs. Since there are neither past nor future parts of time, similarly there are neither past nor future parts of the migration or displacement which is a process, i.e. there is neither past nor future agitation except perhaps if one wishes to take and recognize the space travelled through during the said movement for the existence of the movement itself. They are not the same. The act of moving which is carried out in the travelled space does not exist simultaneously in its totality but occurs successively along the flowing length of the space of time. Therefore, reasonably, the travelled space can be compared with the quantity of time which is measured from this elapsing of time.