396 to spend a little more time discussing this. For I wish to hear from you concerning a question I have.If, then, your opinion is true, as your demonstrations seem to bear out, the contrary opinion will necessarily be false: from this it necessarily follows that Aristotle fell into some error when he tried to demonstrate the contrary; which he did, of course, in Book IV of the <i>Physics</i>.And since he made use of a kind of geometric demonstration, I am surprised that sophisms are found in it: and for that reason I would like to ask you again and again to make this error visible.
AL. If we want to examine with precision Aristotle's demonstration, the discussion will be too long, carrying us beyond our primary aim. But, in accordance with your wish, I will uncover completely the sophisms in it; and so that they may appear more clearly, I will bring forth his demonstration for everyone to see.In the first place, then, he puts this forward: that the slowness and the swiftness of motion depend on a twofold cause; namely, either on the mobile itself, or on the medium itself. On the mobile itself, to be sure: for a heavier mobile will be moved through the same medium more swiftly than a less heavy one.On the other hand, he says that by reason of the medium slowness or swiftness happens in two ways: first when, in the case of the same medium, either it remains at rest, either it rushs in the contrary or in the same directions with the mobile: for the motion of the same mobile will be faster if the medium is carried in the same directions than if it remains at rest and [faster] if it remains at rest than if it is moved in the contrary directions: second, in the case of different media, the mobile shall be swifter when it will be moved through one which is more subtle than a thicker one, as through air than through water. These things having first been pointed out, since he saw that the same weight is carried more swiftly through more subtle media than through thicker ones, he presupposed, in second place, that the speed of motion [in one medium] observes the same ratio to the speed [of motion in another] as the subtlety of the first medium to the subtlety of the other medium. {1} These things having been established, turning to the demonstration he argued as follows: Let mobile a cross medium b in time c; but let it cross a more subtle medium, namely d, in time e: it is manifest that, as the thickness of b is to the thickness of d, thus time c is to time e.Then let f be a void; and let mobile a, if that can happen, cross this f in time g: the void will thus have the same ratio