The Pneumatics of Hero of Alexandria

its motion in itself, but outside of itself. Therefore, things moved with violent motion are moved by a motor that is outside the moved thing, which, when it has then begun to move, is not so much the motor that moves it as the impetus acquired. Just as the heat caused by fire in water, even if the fire is removed, nonetheless burns the hand because the accident united with violence retains its force for some space.

Motion and rest are not opposites, if one considers rest only as a privation, but one motion is indeed contrary to another motion. Everything that moves, either moves all together as a unit, or moves part by part. It is necessary, however, that it have a part in itself at rest, from where the motion takes its principle and upon which the moving part, leaning, can then move what it intends to move. It is necessary that the virtue and force of the motor and of the resting [part] have a certain convenience together, because just as there is a certain force and power by which that which moves is moved, similarly there is a similar power by which that which is said to be at rest stays firm.

The same respect that one considers a motion has toward another motion, a rest will have toward another rest. Similarly, in that degree that motion is found with respect to rest, it is necessary that rest be found with respect to motion. Equal powers do not imprint [themselves] on one another, since the impression happens through the dominance and excellence of the forces.

Whether the vacuum void exists or not has been a great controversy among the ancient philosophers. The Pythagoreans thought that it existed outside the world, and they said that the world had the convenience of breathing in that vacuum and from it. This opinion was also followed by Cleomedes, whence he strove to establish it with many of his arguments, concluding that that nothingness, which some have said is found beyond the heavens, is the same vacuum, which is I know not what, most simple, incorporeal, not comprehensible by the sense, which does not have and cannot receive a figure, and has no power to act or to suffer; but [it] stood absolutely disposed to receive a body. And in sum, the vacuum according to them is that which can be filled or abandoned by a body in the mutations that are made from place to place. But not finding (as was said