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On The Heavens   



call things heavy and light because they have the power of being moved

naturally in a certain way. The activities corresponding to these

powers have not been given any name, unless it is thought that

'impetus' is such a name. But because the inquiry into nature is

concerned with movement, and these things have in themselves some

spark (as it were) of movement, all inquirers avail themselves of

these powers, though in all but a few cases without exact

discrimination. We must then first look at whatever others have

said, and formulate the questions which require settlement in the

interests of this inquiry, before we go on to state our own view of

the matter.

Language recognizes (a) an absolute, (b) a relative heavy and light.

Of two heavy things, such as wood and bronze, we say that the one is

relatively light, the other relatively heavy. Our predecessors have

not dealt at all with the absolute use, of the terms, but only with

the relative. I mean, they do not explain what the heavy is or what

the light is, but only the relative heaviness and lightness of

things possessing weight. This can be made clearer as follows. There

are things whose constant nature it is to move away from the centre,

while others move constantly towards the centre; and of these

movements that which is away from the centre I call upward movement

and that which is towards it I call downward movement. (The view,

urged by some, that there is no up and no down in the heaven, is

absurd. There can be, they say, no up and no down, since the

universe is similar every way, and from any point on the earth's

surface a man by advancing far enough will come to stand foot to

foot with himself. But the extremity of the whole, which we call

'above', is in position above and in nature primary. And since the

universe has an extremity and a centre, it must clearly have an up and

down. Common usage is thus correct, though inadequate. And the

reason of its inadequacy is that men think that the universe is not

similar every way. They recognize only the hemisphere which is over

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