The Pneumatics of Hero of Alexandria

above; and as Aristotle holds) a place without a body, nor a clear thing that such an opinion is false. Others wanted it to be found and that the vacuum could stay absolutely amassed in various places within the ambit and circle of the heavens, asserting that if this were not true, motion from place to place would be annulled. For since there is some space from the place where the thing moves to where it intends to arrive, if that space were not vacant but filled with another body, either that body, by opposing, would impede the movement, or multiple bodies would penetrate one another, or, by yielding, it would be necessary that other contiguous bodies make place for the one that yields; and thus the others from hand to hand reciprocally, until one arrived at the circle of the heavens, which is the ultimate place; all things impossible. And Zeno and Melissus tangled themselves so much in these opinions that they lent more faith to the arguments than to the sense.

But how a body yields and gives place to another, and how condensation and rarefaction are made, not by respect of the vacuum, Aristotle taught highly in text 63 and 84 of the fourth book of the Physics, saying, because some rarer bodies, by compression, are driven out of the compressed body, or they re-enter the body that dilates, or because the property and nature of matter is such that, being in potency, it can reduce itself to act and receive a greater and smaller quantity, without anything else being added to it from outside. With these foundations, it is easy to dissolve all the reasons leaning on the sense to prove the vacuum. But in order not to be out of mode long, and because it is not our end to treat such a matter in this place, I will leave it to others to see it in the proper sources of Aristotle, and also of the Interpreters, who treated all this diffusely in the 4th [book] of the Physics from text 50 to 86. I will say well that not only is it not true that to make movement from place to place it is necessary to concede the vacuum, but furthermore, that by conceding it, local motion would come to be annulled. For since this happens either faster or slower according to the disposition of the medium through which it is made, if the medium were therefore vacuum, no contrast at all would be found in the motion, such that it would be made not only very fast, but in an instant. In an instant, it cannot be made, which