318 splitting that the swiftness of motion depends. Now there is no one who can ignore how incongruous all these things are.
But, leaving aside the ways of thinking of the others, in order that we may track down what we believe to be the true cause of this effect, we will make use of the following resolutive {1} method. Since, then, a heavy mobile (let us however speak about natural downward motion, coming from heaviness: for, that being known, we will judge the case of upward motion by proceeding in reverse) in going down is moved more slowly at the beginning, it is therefore necessary that it be less heavy at the beginning of its motion than in the middle or at the end; for we know with certainty, from the things demonstrated in the first book {1}, that speed and slowness follow heaviness and lightness. If, then, it is found out how and why a mobile is less heavy at the beginning, the cause for which it goes down more slowly will certainly have been found. But the natural and intrinsic heaviness of the mobile is certainly not diminished, since neither its size nor its density is diminished: it remains, therefore, that that diminution of heaviness is against nature and accidental. Hence if we have found in what way the heaviness of the mobile is diminished against nature and extrinsically, what we need will surely have been found. But that heaviness is not diminished by the heaviness of the medium, for the medium is the same at the beginning of motion as at the middle: it remains, therefore, that the heaviness of the mobile is diminshed by some violence that is extrinsic and comes from outside (for it is only in these two ways that a mobile gets to be light by accident). {1} If then, again, we find out how a mobile could be lifted by an extrinsic force, the cause of slowness, again, will have been found. Now the force impressed by a thrower not only at times diminishes the heaviness of a heavy thing, but it often even renders it so light that it flies upward with great speed: hence let us see and search attentively, whether perhaps this force is the cause of the diminishing of the heaviness of the mobile at the beginning of its motion. And, I say, it certainly is that force impressed by the thrower which renders natural motion weaker at the beginning: so let us hasten to make clear by what method