283 : for I will say the same thing concerning air; for when a stone is in the air, how does it give way more downward than upward, or to the left rather than to the right, if the rariness of the air is everywhere the same?Here perhaps someone might say, following Aristotle {1}, that air exerts weight in its own region, and because of that it helps downward motion more: but we will examine these chimaeras in the next chapter, where we will inquire whether the elements exert weight in their proper place. Similarly also when they say {1}, In the void there is no upward nor downward; who has dreamt this up? Is it not the case that, if the air were a void, the void in the vicinity of earth would be nearer to the center than the void which would be in the vicinity of fire? Further, as to the argument Aristotle makes concerning projectiles, saying: Projectiles cannot be moved in the void, for projectiles, when they are beyond the hand of the mover, are moved by the air or by another corporeal medium, which surrounds them and is moved, which is surely lacking in the void; this is similarly of no importance whatsoever: for he assumes that projectiles are borne by the medium; that this is false, we will demonstrate in its proper place. {1} Similarly false is what he adds {1} to the argument, concerning different mobiles in the same medium: for he assumes that in a plenum heavier things are carried faster, because they cleave the medium more strongly, and that this is the only cause of the swiftness; since this resistance does not exist in a void , he infers that all motions in the void will be in the same time and with the same swiftness {2}: and this he asserts to be impossible.And yet, in the first place, Aristotle sins in this, that he does not show how it is absurd, that in the void different mobiles are moved with the same swiftness: but he sins even more when he assumes that the swiftnesses of motions of different mobiles depend on the fact that heavier mobiles divide the medium better. For the swiftness of mobiles is not to be sought from that, as has been demonstrated above, but from the greater excess of the heaviness of mobiles over the heaviness of the medium; for the swiftnesses follow the ratio of such excesses: but the excess of the heaviness of different mobiles over the heaviness of the same medium is not the same (for the mobiles would be equally heavy): thus neither will the swiftnesses be equal. Thus in the case of a mobile whose heaviness is 8, the excess over the heaviness of the void, which is null, is 8; thus its swiftness will be 8: but the excess of a mobile whose heaviness is 4 over the void will similarly be 4; and thus its swiftness is 4. Finally, by using in the case of the void the same demonstration that we have put forward in
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