determined to call it 'ether'.

  For the um opinions appear in cycles among men not once nor twice,
but infinitely often.

  Now there are some who maintain that not only the bodies in motion
but that which contains them is pure fire, and the interval between
the earth and the stars air: but if they had considered what is now
satisfactorily established by mathematics, they might have given up
this puerile opinion. For it is altogether childish to suppose that
the moving bodies are all of them of a small size, because they so
to us, looking at them from the earth.

  This a matter which we have already discussed in our treatment of
the upper region, but we may return to the point now.

  If the intervals were full of fire and the bodies consisted of
fire every one of the other elements would long ago have vanished.

  However, they cannot simply be said to be full of air either; for
even if there were two elements to fill the space between the earth
and the heavens, the air would far exceed the quantitu required to
maintain its proper proportion to the other elements. For the bulk
of the earth (which includes the whole volume of water) is
infinitesimal in comparison with the whole world that surrounds it.
Now we find that the excess in volume is not proportionately great
where water dissolves into air or air into fire. Whereas the
proportion between any given small quantity of water and the air
that is generated from it ought to hold good between the total
amount of air and the total amount of water. Nor does it make any
difference if any one denies that the elements originate from one
another, but asserts that they are equal in power. For on this view it
is certain amounts of each that are equal in power, just as would be
the case if they actually originated from one another.

  So it is clear that neither air nor fire alone fills the
intermediate space.

  It remains to explain, after a preliminary discussion of
difficulties, the relation of the two elements air and fire to the
position of the first element, and the reason why the stars in the
upper region impart heat to the earth and its neighbourhood. Let us
first treat of the air, as we proposed, and then go on to these
questions.

  Since water is generated from air, and air from water, why are
clouds not formed in the upper air? They ought to form there the more,
the further from the earth and the colder that region is. For it is
neither appreciably near to the heat of the stars, nor to the rays
relected from the earth. It is these that dissolve any formation by
their heat and so prevent clouds from forming near the earth. For
clouds gather at the point where the reflected rays disperse in the
infinity of space and are lost. To explain this we must suppose either
that it is not all air which water is generated, or, if it is produced
from all air alike, that what immediately surrounds the earth is not
mere air, but a sort of vapour, and that its vaporous nature is the
reason why it condenses back to water again. But if the whole of
that vast region is vapour, the amount of air and of water will be
disproportionately great. For the spaces left by the heavenly bodies
must be filled by some element. This cannot be fire, for then all
the rest would have been dried up. Consequently, what fills it must be
air and the water that surrounds the whole earth-vapour being water
dissolved.

  After this exposition of the difficulties involved, let us go on
to lay down the truth, with a view at once to what follows and to what
has already been said. The upper region as far as the moon we affirm
to consist of a body distinct both from fire and from air, but varying
degree of purity and in kind, especially towards its limit on the side
of the air, and of the world surrounding the earth. Now the circular
motion of the first element and of the bodies it contains dissolves,
and inflames by its motion, whatever part of the lower world is
nearest to it, and so generates heat. From another point of view we
may look at the motion as follows. The body that lies below the
circular motion of the heavens is, in a sort, matter, and is
potentially hot, cold, dry, moist, and possessed of whatever other
qualities are derived from these. But it actually acquires or
retains one of these in virtue of motion or rest, the cause and
principle of which has already been explained. So at the centre and
round it we get earth and water, the heaviest and coldest elements, by
themselves; round them and contiguous with them, air and what we
commonly call fire. It is not really fire, for fire is an excess of
heat and a sort of ebullition; but in reality, of what we call air,
the part surrounding the earth is moist and warm, because it
contains both vapour and a dry exhalation from the earth. But the next
part, above that, is warm and dry. For vapour is naturally moist and
cold, but the exhalation warm and dry; and vapour is potentially
like water, the exhalation potentially like fire. So we must take
the reason why clouds are not formed in the upper region to be this:
that it is filled not with mere air but rather with a sort of fire.

  However, it may well be that the formation of clouds in that upper
region is also prevented by the circular motion. For the air round the
earth is necessarily all of it in motion, except that which is cut off
inside the circumference which makes the earth a complete sphere. In
the case of winds it is actually observable that they originate in
marshy districts of the earth; and they do not seem to blow above
the level of the highest mountains. It is the revolution of the heaven
which carries the air with it and causes its circular motion, fire
being continuous with the upper element and air with fire. Thus its