285 Chapter [11] In which Aristotle's error, in saying, that air exerts weight in its own place, is made manifest.
The method which we shall observe in this treatise will be that the things that must be said always depend on those that have been said; and that (as much as this will be possible) I never presuppose as true those that must be made clear. As a matter of fact my masters in mathematics have taught me this method: but it is not sufficently observed by certain philosophers {1}, who quite often, in teaching the elements of physics, presuppose things that have been reported either in the books De Anima, or in the books De Caelo, and even in the Metaphysics; and not only that, but even, in teaching logic itself, they constantly mouth words that have been reported in the last books of Aristotle; so that, while they teach pupils the first rudiments, they presuppose that these pupils know everything, and they hand down {2} their teaching not from things better known, but from things purely and simply unknown and unheard of. Now what happens to those who learn this way is that they never know anything by its causes, but they only believe as by faith, that is because Aristotle has said so. There are only a few who inquire whether what Aristotle said is true: for it suffices for them that they will have the reputation of being more learned, the more passages of Aristotle they have at hand. But, leaving this aside, returning to our subject, it must be considered whether air and water really have weight in their proper places: for this question can be explained presupposing only the things that have been reported
Aristotle has written in Book IV of the De Caelo, text #30 [311b8-10] {1} that not only is water heavy in its own place, but air also: saying that, except for fire, everything has weight in its own region, even air itself.Now concerning air he immediatly verifies this with the aid of a sign; saying, since a bladder pulls by force more when inflated than when it is not, this is thus a sign that the air in the bladder has weight.He repeats the same thing in text #39 of the same Book [312b2-19], saying that all things that have heaviness and lightness, have heaviness in their own region: for he assumes that air and water in relation to the other elements are sometimes heavy yet sometimes light, but they exert weight absolutely only in their proper region.Now some more recent philosophers {1}, having noted what Aristotle has said in Book III of the De Caelo, text #28 [301b22-26] {2}, namely that air helps both motions, that is, to the extent it is light, it helps motion