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On The Heavens   

to the surface of all things-and we observe fire to move upward even

in air itself, while the air remains at rest-clearly this body is

moving towards the extremity. It cannot then have any weight. If it

had, there would be another body in which it sank: and if that had

weight, there would be yet another which moved to the extremity and

thus rose to the surface of all moving things. In fact, however, we

have no evidence of such a body. Fire, then, has no weight. Neither

has earth any lightness, since it sinks to the bottom of all things,

and that which sinks moves to the centre. That there is a centre

towards which the motion of heavy things, and away from which that

of light things is directed, is manifest in many ways. First,

because no movement can continue to infinity. For what cannot be can

no more come-to-be than be, and movement is a coming to-be in one

place from another. Secondly, like the upward movement of fire, the

downward movement of earth and all heavy things makes equal angles

on every side with the earth's surface: it must therefore be

directed towards the centre. Whether it is really the centre of the

earth and not rather that of the whole to which it moves, may be

left to another inquiry, since these are coincident. But since that

which sinks to the bottom of all things moves to the centre,

necessarily that which rises to the surface moves to the extremity

of the region in which the movement of these bodies takes place. For

the centre is opposed as contrary to the extremity, as that which

sinks is opposed to that which rises to the surface. This also gives a

reasonable ground for the duality of heavy and light in the spatial

duality centre and extremity. Now there is also the intermediate

region to which each name is given in opposition to the other extreme.

For that which is intermediate between the two is in a sense both

extremity and centre. For this reason there is another heavy and

light; namely, water and air. But in our view the continent pertains

to form and the contained to matter: and this distinction is present

in every genus. Alike in the sphere of quality and in that of quantity

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