anything can
be moved without a mover is hard or indeed impossible, and equally
impossible to conceive that there can be a mover unless there be
something which can be moved--motion cannot exist where either of these
are wanting, and for these to be uniform is impossible; wherefore we
must assign rest to uniformity and motion to the want of uniformity. Now
inequality is the cause of the nature which is wanting in uniformity;
and of this we have already described the origin. But there still
remains the further point--why things when divided after their kinds do
not cease to pass through one another and to change their place--which
we will now proceed to explain. In the revolution of the universe are
comprehended all the four elements, and this being circular and having a
tendency to come together, compresses everything and will not allow any
place to be left void. Wherefore, also, fire above all things penetrates
everywhere, and air next, as being next in rarity of the elements;
and the two other elements in like manner penetrate according to their
degrees of rarity. For those things which are composed of the largest
particles have the largest void left in their compositions, and those
which are composed of the smallest particles have the least. And the
contraction caused by the compression thrusts the smaller particles into
the interstices of the larger. And thus, when the small parts are placed
side by side with the larger, and the lesser divide the greater and the
greater unite the lesser, all the elements are borne up and down and
hither and thither towards their own places; for the change in the size
of each changes its position in space. And these causes generate an
inequality which is always maintained, and is continually creating a
perpetual motion of the elements in all time.

In the next place we have to consider that there are divers kinds
of fire. There are, for example, first, flame; and secondly, those
emanations of flame which do not burn but only give light to the eyes;
thirdly, the remains of fire, which are seen in red-hot embers after the
flame has been extinguished. There are similar differences in the air;
of which the brightest part is called the aether, and the most turbid
sort mist and darkness; and there are various other nameless kinds which
arise from the inequality of the triangles. Water, again, admits in the
first place of a division into two kinds; the one liquid and the other
fusile. The