As mentioned above, it is obvious that some bodies are moved by an external and separate motor. One usually searches for what this moving cause does essentially or what effect it provokes in nature. We observe that bodies which move, are impelled, sustain a blow from an external motor, not only are agitated by a movement while being connected to the moving body itself and are led by the latter as if it were by a hand, but, even after withdrawal of the external mover, the moved bodies themselves, left without a guide, continue the movement which was begun. They thus seem to have acquired instruction, discipline, and even force and faculty to be able to complete the movement. Since the subsequent movement also results from another cause rather than from nothing, it is commonly inferred that the cause and the motive force by which the subsequent movement can be completed and continued, remain in the projectile after this was left by the impeller. A force and some sort of motive faculty have been assumed. If it had been impressed by the projecting body in the moved body, an occurring motive quality would of course migrate from a subject into another. This is commonly considered to be absurd. People then have imagined a new impelling cause which is nothing else than the fluid environment itself, such as water or air which is the motor replacing the projecting subject. Therefore, after grasping the mobile, this fluid continues the previous movement by transporting it until, with the extinction of the motive faculty of air, the movement of the projectile is brought to an end. Although this concept is refuted with the strongest arguments by others and mainly by Galileo, I usefully showed the error of this opinion with new arguments which I found by thinking.

It is usual to defend the peripatetic claim in two ways. For some people, a stone, for instance, is thrown by the hand which holds it. The hand, before parting from the stone, propels the ambient air as well as the stone. When no longer held by the hand, deprived of its propulsion, the stone which has by itself no motive force would fall perpendicularly to the surface of the earth as a result of its weight. But it is surrounded everywhere by air previously agitated and set in movement. Therefore, the stone is carried by the ambient air as if it is were by a rapid stream, until the agitation of the air weakens and finally ceases together with the movement of the carried stone. Others belonging to the same peripatetic school say that the air following the projectile, by returning at the highest velocity and with force to refill the space left by the projectile, to avoid the creation of vacuum abhorrent to nature, moves the stone forwards by pressing continuously behind most violently. This is similar to children who propel plum-stones by pinching them between their fingers.

The falsity of this concept will be shown. If the fluid environment, such as water or air, carries the projected body like a stream, after it has parted from the projecting subject, one must admit that air or water was also impelled by the same projecting subject. Such fluid environment is also mobile and has been subjected to a movement of which it was deprived previously. Consequently, when the propulsion by the projecting subject ceases, except if we are willing to accept a migration of the motive faculty from one subject to another, i.e. from the projecting subject to the fluid itself, we are forced to determine the cause of the continuation of the subsequent movement of this fluid itself. Therefore, some other fluid body surrounding the air or the water so far projected and impelled must be found by which the flow can continue and carry with it the included projectile. This external fluid itself should also be impelled by another ambient fluid and so on ad infinitum. The stone, therefore, could not be projected in the air without an agitation of the whole world. Obviously this does not occur and is impossible. A projection is carried out in a bottle full of water and closed hermetically. If the stone was moved forwards by the ambient water after being parted from the projecting subject, although it is admitted that all the water is moved and whirls up to the surface of the container, the external surface of the water contiguous to the glass, if it was deprived of motive virtue, would have to be carried and propelled by the surface of the glass itself which is absolutely immobile. This concept thus is erroneous. Moreover, it is obvious that air or water are little disturbed and not at all moved by a projectile. They remain very quiet. This appears most clearly in water partially tainted by some coloured powder. Then the movement of the water itself can be observed by the displacement of the corpuscles floating in it. At some distance from the projected body they appear to be absolutely at rest. You will observe the same phenomenon in air after choosing a convenient place where the movement and agitation of the dust fluttering in air can be seen. This occurs in a closed room in which sun rays entering through a window make visible the dust fluttering in the air. There the atoms are not agitated by the projectile at a short distance from the projection. Even more, if a projectile falls on a pointed and stable spear-head or is retained by a very thin thread, the reflection of the projectile occurs along its line of incidence. At the same time, the corpuscles floating in the air or in the water and which were contiguous to the projectile, and the ones which surrounded the projectile closely pursue their first movement straight on. It is thus conjectured that the fluid also agitated by the same direct movement cannot generate a movement opposite to the reflection of the projectile, i.e. it cannot carry the projectile backwards while the fluid runs and proceeds forwards.

This appears even more clearly from our arguments against the other concept which invokes the horror of vacuum. The reason why these absurd concepts are retained by some obstinate people is that they acquiesce to the words while ignoring, not perceiving or considering carelessly the facts. If they were more perspicuous, no doubt they would reject this concept. Therefore, it is worthwhile explaining clearly all the problems of the said peripatetic concept.

Unquestionably the fluid in which a projectile moves either undergoes neither condensation nor rarefaction by the nature of such water or it seems to be made of quicksilver, or it can be constricted or dilated in an utmost manner like air.