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On The Heavens   

lightness, must be moved by constraint, and must continue its

constrained movement infinitely. For there will be a force which moves

it, and the smaller and lighter a body is the further will a given

force move it. Now let A, the weightless body, be moved the distance

CE, and B, which has weight, be moved in the same time the distance

CD. Dividing the heavy body in the proportion CE:CD, we subtract

from the heavy body a part which will in the same time move the

distance CE, since the whole moved CD: for the relative speeds of

the two bodies will be in inverse ratio to their respective sizes.

Thus the weightless body will move the same distance as the heavy in

the same time. But this is impossible. Hence, since the motion of

the weightless body will cover a greater distance than any that is

suggested, it will continue infinitely. It is therefore obvious that

every body must have a definite weight or lightness. But since

'nature' means a source of movement within the thing itself, while a

force is a source of movement in something other than it or in

itself qua other, and since movement is always due either to nature or

to constraint, movement which is natural, as downward movement is to a

stone, will be merely accelerated by an external force, while an

unnatural movement will be due to the force alone. In either case

the air is as it were instrumental to the force. For air is both light

and heavy, and thus qua light produces upward motion, being

propelled and set in motion by the force, and qua heavy produces a

downward motion. In either case the force transmits the movement to

the body by first, as it were, impregnating the air. That is why a

body moved by constraint continues to move when that which gave the

impulse ceases to accompany it. Otherwise, i.e. if the air were not

endowed with this function, constrained movement would be

impossible. And the natural movement of a body may be helped on in the

same way. This discussion suffices to show (1) that all bodies are

either light or heavy, and (2) how unnatural movement takes place.

From what has been said earlier it is plain that there cannot be

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