397 to a plenum as g has to e.Let us then consider another medium more subtle than d, to whose thickness the thickness of d has the same ratio as time e has to time g.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 unacceptable.
This is Aristotle's demonstration: to be sure it would have reached a sound and necessary conclusion, as far as the form of the demonstration is concerned {1}, 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 Aristotle has (in my opinion) been deceived in this, that he assumed things as known axioms which not only are not known to the senses, but have never been demonstrated either, nor are they even demonstrable, since they are completely false. In order that this may appear clearer than daylight, we will examine his hypotheses one by one.
First, then, he assumes that the cause of the swiftness and the slowness of the motion is the subtlety or the thickness of the medium, through which the motion takes place: this supposition is assuredly false, both because of what has been demonstrated above {1}; where it was shown that it is the heaviness of the medium, and not its thickness or its subtlety, which is the cause of the slowness or swiftness of motion; and also because of the things that we shall add now. For, I pray you, if the subtlety of the medium is the cause of the swiftness of the motion, no doubt in a more subtle medium motion will take place faster: now, according to Aristotle himself, air is more subtle than water: yet certain things are moved faster in water than in air. {1}Just as if, for example, we take an inflated bladder, it will be moved with natural motion more slowly in air than in water: for if it were restrained and tied down in deep water, then, released from its bonds, the bladder would fly very swiftly upward; now if we took a still lighter body, it would be moved more slowly in air, but faster in water; so that we would be able to arrive at something which in air would hardly be moved, but in water [would be moved] very swiftly. Moreover I will add this: for if we consider a certain body so light that it goes up in air, no doubt it will be lighter than the bladder; if then such a body is held by force under water, and is then released, who will doubt that it will go up far more swiftly in water than