Now all unmixed and primary bodies are produced by such causes as
these. As to the subordinate species which are included in the
greater kinds, they are to be attributed to the varieties in the
structure of the two original triangles. For either structure did
not originally produce the triangle of one size only, but some
larger and some smaller, and there are as many sizes as there are
species of the four elements. Hence when they are mingled with
themselves and with one another there is an endless variety of
them, which those who would arrive at the probable truth of
nature ought duly to consider.
Unless a person comes to an understanding about the nature and
conditions of rest and motion, he will meet with many
difficulties in the discussion which follows. Something has been
said of this matter already, and something more remains to be
said, which is, that motion never exists in what is uniform. For
to conceive that 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