what is its position relatively to the other physical elements. (For
there is no question as to the relation of the bulk of the earth to
the size of the bodies which exist around it, since astronomical
demonstrations have by this time proved to us that it is actually
far smaller than some individual stars. As for the water, it is not
observed to exist collectively and separately, nor can it do so
apart from that volume of it which has its seat about the earth: the
sea, that is, and rivers, which we can see, and any subterranean water
that may be hidden from our observation.) The question is really about
that which lies between the earth and the nearest stars. Are we to
consider it to be one kind of body or more than one? And if more
than one, how many are there and what are the bounds of their regions?
We have already described and characterized the first element, and
explained that the whole world of the upper motions is full of that
This is an opinion we are not alone in holding: it appears to be
an old assumption and one which men have held in the past, for the
word ether has long been used to denote that element. Anaxagoras, it
is true, seems to me to think that the word means the same as fire.
For he thought that the upper regions were full of fire, and that
men referred to those regions when they spoke of ether. In the
latter point he was right, for men seem to have assumed that a body
that was eternally in motion was also divine in nature; and, as such a
body was different from any of the terrestrial elements, they
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