398 afterwards in air, although air is more subtle than water?How, then, will it ever be true that natural motion is necessarily faster in a more subtle medium than in a thicker one? {1}
Now here it does not escape me that there is a great number of more recent philosophers who profess to know things which they know more by relying on the authority of others than by demonstration: these persons, if they heard such things, would immediately undertake to reply, and they would be satisfied to throw in a couple of words, even if they had nothing to do with the subject; for a little latter they would add, Such an opinion has been more than sufficiently and fully refuted before; and it is with similar turgid terms that they persuade themselves and their listeners, who are even more ignorant than they, of their opinions.For to these people, if they were to hear my arguments, and retorted that my reasoning is not conclusive, because I speak now of upward motion and now of downward, which is contrary to Aristotle's intention, or brought forward similar remarks without any soundness, {1} to these people, I say, it would seem that my opinion has been more than sufficiently refuted. But so much for them: let it be sufficient for me that I would have forestalled such a response of theirs, when I added a second example {1}, which only talks of one motion.It must therefore be concluded that the following is entirely false, namely that the slowness or the swiftness of motion results from the thickness or the subtlety of the medium.
Secondly: Aristotle has taken for granted as if it were known, that the speed of one motion has the same ratio to the speed of a second motion, as the subtlety of the first medium has to the subtlety of the second medium. This, Aristotle has not demonstrated, and he even skillfully avoided it: for to have taken pains to do so would have been in vain, since it is not demonstrable, and, not only not demonstrable, but actually false. For, even if it is conceded that subtlety is the cause of swiftness, it will follow, to be sure, that in a [medium of] greater subtlety, the swiftness will be greater; but it still will not follow that the swiftnesses and the subtleties increase in the same ratio. And, to use Aristotle's way of speaking, the subtlety of air would have no ratio to the subtlety of water: for, as an example, {1} wood goes down in air, but not in water; hence the swiftness in air will have no ratio to the swiftness in water. {2}In order that this may appear clearer than daylight,