timaeus   

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 liquid kind is
composed of the small and unequal particles of water; and moves itself
and is moved by other bodies owing to the want of uniformity and the
shape of its particles; whereas the fusile kind, being formed of large
and uniform particles, is more stable than the other, and is heavy and
compact by reason of its uniformity. But when fire gets in and
dissolves the particles and destroys the uniformity, it has greater
mobility, and becoming fluid is thrust forth by the neighbouring air
and spreads upon the earth; and this dissolution of the solid masses
is called melting, and their spreading out upon the earth flowing.
Again, when the fire goes out of the fusile substance, it does not
pass into vacuum, but into the neighbouring air; and the air which
is displaced forces together the liquid and still moveable mass into
the place which was occupied by the fire, and unites it with itself.
Thus compressed the mass resumes its equability, and is again at unity
with itself, because the fire which was the author of the inequality
has retreated; and this departure of the fire is called cooling, and
the coming together which follows upon it is termed congealment. Of
all the kinds termed fusile, that which is the densest and is formed
out of the finest and most uniform parts is that most precious
possession called gold, which is hardened by filtration through
rock; this is unique in kind, and has both a glittering and a yellow
colour. A shoot of gold, which is so dense as to be very hard, and
takes a black colour, is termed adamant. There is also another kind
which has parts nearly like gold, and of which there are several
species; it is denser than gold, and it contains a small and fine
portion of earth, and is therefore harder, yet also lighter because of
the great interstices which it has within itself; and this