284 the case of the plenum, we will demonstrate that mobiles of the same species, but of different size, are moved with the same swiftness in the void.And that is enough on this subject.
The {1} force of truth is such, that the most learned men, and even Peripatetics, have recognized the fallacy of this way of thinking of Aristotle, although none of them has been able to refute Aristotle's argument in an appropriate way.And certainly none of them has ever been able to knock down the argument which is written in Book IV of the Physics, texts #71 and #72 [215a24 (-215b22)-216a3]: for up to now the fallacy in it has never been observed: and although Scotus, the divine Thomas, Philoponus {1} and some others hold a way of thinking contrary to Aristotle, they have, however, arrived at the truth by belief more than via true demonstration, or by having answered Aristotle. And, as a matter of fact, there is not one of them who could hope to be able to answer Aristotle and knock down his demonstration, if he concedes the ratio which is assumed by him between the speeds of the same mobile in different media. For he assumes that the speed in one medium is so related to the speed in another, as the subtlety of one medium to the subtlety in the other: this no one up to now has dared to deny. {1} Nor is there any soundness in what is assumed by the authors mentioned above, namely a double resistance of the mobile to motion -- one, extrinsic, coming from the thickness of the medium; the other, intrinsic, by reason of the determinate heaviness of the mobile. For this is somehow a fictitious thing: for if we consider the thing accurately, these two resistances do not differ from one another. For, as has been made clear above, the thicknesses or ( to better express myself) the heaviness of the medium makes the lightness of the mobile, and the lightness of the medium provides the heaviness of the mobile; and the same mobile is now heavier now lighter, according as it is in a lighter or a heavier medium. Consequently they add nothing new by assuming this double resistance; since it is only increased and diminished according to the diminution or the increase of the heaviness or the thickness of the medium. On the other hand, if they concede that it is increased and diminished in the ratio in which the heavinesses of the medium vary, it is in vain that they will try to knock down Aristotle's argument.