that, if a void were given, motion in it would take place in an instant: and since we are just about to determine that, in a void motion takes place in time, we will examine first the contrary opinion and its arguments. </s>
</p>
<p>
<s id="id.1.1.10.02.00"/>
<s id="id.1.1.10.02.01">Now, first of all, of the arguments alleged by Aristotle, there is surely none that has necessity; there is however one which, at first sight, seems to have necessity: and this is the one which is written in texts #71 and 72 [215a 24 (-215b 22) - 216a 7], where he deduces the following unacceptable consequence, namely that, if motion in the void took place in time, the same mobile will be moved in a plenum and in a void in the same time. </s>
<s id="id.1.1.10.02.02">In order that we may be better able to refute this argument, we have decided now to bring it forward for everyone to see. </s>
<s id="id.1.1.10.02.03">Thus, first, when he saw that the same mobile is carried more swiftly through more subtle media than through thicker ones, Aristotle assumed this: the ratio of the speed of the motion in one medium to the speed of the other motion in the other medium is the same as the ratio of the subtlety of one medium to the sublety of the other. </s>
</p>
<p>
<s id="id.1.1.10.03.00.fig"/>
<s id="id.1.1.10.03.01">He then 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.</s>
<s id="id.1.1.10.03.02">Then let f be a void; and let mobile a, if that can happen, cross this f, not in an instant, but in time g; and let the thickness of medium d be to the thickness of another medium as time e is to time g.</s>
<s id="id.1.1.10.03.03">Now, from the things that have been established, mobile a will be moved through this medium that has just been found in time g, since medium d has the same ratio to this medium that has just been found as time e to time g; but, in this same time g, a is also moved through void f: hence a, in the same time, will be moved through two equal distances, one of which is a plenum, but the other a void; which assuredly is impossible. </s>
<s id="id.1.1.10.03.04">Therefore the mobile will not be moved through the void in time; therefore [the motion will take place] in an instant. </s>
</p>
<p>
<s id="id.1.1.10.04.00"/>
<s id="id.1.1.10.04.01">This is Aristotle's demonstration: to be sure it would have concluded very much to the point and from necessity, if Aristotle had demonstrated the things he took for granted, or, if they had not been demonstrated, if they had at least been true ; but he has been deceived {1} in this, that these things, which he took for granted as well known axioms things which are not only not </s>
