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of matter, then, must be as numerous as these bodies, i.e. four, but
though they are four there must be a common matter of all-particularly
if they pass into one another-which in each is in being different.
There is no reason why there should not be one or more intermediates
between the contraries, as in the case of colour; for 'intermediate'
and 'mean' are capable of more than one application.
Now in its own place every body endowed with both weight and lightness
has weightwhereas earth has weight everywhere-but they only have lightness
among bodies to whose surface they rise. Hence when a support is withdrawn
such a body moves downward until it reaches the body next below it,
air to the place of water and water to that of earth. But if the fire
above air is removed, it will not move upward to the place of fire,
except by constraint; and in that way water also may be drawn up,
when the upward movement of air which has had a common surface with
it is swift enough to overpower the downward impulse of the water.
Nor does water move upward to the place of air, except in the manner
just described. Earth is not so affected at all, because a common
surface is not possible to it. Hence water is drawn up into the vessel
to which fire is applied, but not earth. As earth fails to move upward,
so fire fails to move downward when air is withdrawn from beneath
it: for fire has no weight even in its own place, as earth has no
lightness. The other two move downward when the body beneath is withdrawn
because, while the absolutely heavy is that which sinks to the bottom
of all things, the relatively heavy sinks to its own place or to the
surface of the body in which it rises, since it is similar in matter
to it.
It is plain that one must suppose as many distinct species of matter
as there are bodies. For if, first, there is a single matter of all
things, as, for instance, the void or the plenum or extension or the
triangles, either all things will move upward or all things will move
downward, and the second motion will be abolished. And so, either
there will be no absolutely light body, if superiority of weight is
due to superior size or number of the constituent bodies or to the
fullness of the body: but the contrary is a matter of observation,
and it has been shown that the downward and upward movements are equally
constant and universal: or, if the matter in question is the void
or something similar, which moves uniformly upward, there will be
nothing to move uniformly downward. Further, it will follow that the
intermediate bodies move downward in some cases quicker than earth:
for air in sufficiently large quantity will contain a larger number
of triangles or solids or particles. It is, however, manifest that
no portion of air whatever moves downward. And the same reasoning
applies to lightness, if that is supposed to depend on superiority
of quantity of matter. But if, secondly, the kinds of matter are two,
it will be difficult to make the intermediate bodies behave as air
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ON THE HEAVENS 64
and water behave. Suppose, for example, that the two asserted are
void and plenum. Fire, then, as moving upward, will be void, earth,
as moving downward, plenum; and in air, it will be said, fire preponderates,
in water, earth. There will then be a quantity of water containing
more fire than a little air, and a large amount of air will contain
more earth than a little water: consequently we shall have to say
that air in a certain quantity moves downward more quickly than a
little water. But such a thing has never been observed anywhere. Necessarily,
then, as fire goes up because it has something, e.g. void, which other
things do not have, and earth goes downward because it has plenum,
so air goes to its own place above water because it has something
else, and water goes downward because of some special kind of body.
But if the two bodies are one matter, or two matters both present
in each, there will be a certain quantity of each at which water will
excel a little air in the upward movement and air excel water in the
downward movement, as we have already often said.
Part 6
The shape of bodies will not account for their moving upward or downward
in general, though it will account for their moving faster or slower.
The reasons for this are not difficult to see. For the problem thus
raised is why a flat piece of iron or lead floats upon water, while
smaller and less heavy things, so long as they are round or long-a
needle, for instance-sink down; and sometimes a thing floats because
it is small, as with gold dust and the various earthy and dusty materials
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