timaeus   

which has the fewest bases must necessarily be the most moveable,
for it must be the acutest and most penetrating in every way, and also
the lightest as being composed of the smallest number of similar
particles: and the second body has similar properties in a second
degree, and the third body in the third degree. Let it be agreed,
then, both according to strict reason and according to probability,
that the pyramid is the solid which is the original element and seed
of fire; and let us assign the element which was next in the order
of generation to air, and the third to water. We must imagine all
these to be so small that no single particle of any of the four
kinds is seen by us on account of their smallness: but when many of
them are collected together their aggregates are seen. And the
ratios of their numbers, motions, and other properties, everywhere
God, as far as necessity allowed or gave consent, has exactly
perfected, and harmonised in due proportion.
From all that we have just been saying about the elements or
kinds, the most probable conclusion is as follows:-earth, when meeting
with fire and dissolved by its sharpness, whether the dissolution take
place in the fire itself or perhaps in some mass of air or water, is
borne hither and thither, until its parts, meeting together and
mutually harmonising, again become earth; for they can never take
any other form. But water, when divided by fire or by air, on
reforming, may become one part fire and two parts air; and a single
volume of air divided becomes two of fire. Again, when a small body of
fire is contained in a larger body of air or water or earth, and
both are moving, and the fire struggling is overcome and broken up,
then two volumes of fire form one volume of air; and when air is
overcome and cut up into small pieces, two and a half parts of air are
condensed into one part of water. Let us consider the matter in
another way. When one of the other elements is fastened upon by
fire, and is cut by the sharpness of its angles and sides, it
coalesces with the fire, and then ceases to be cut by them any longer.
For no element which is one and the same with itself can be changed by
or change another of the same kind and in the same state. But so
long as in the process of transition the weaker is fighting against
the stronger, the dissolution continues. Again, when a few small
particles, enclosed in many larger ones, are in process of
decomposition and extinction, they only cease from their tendency to
extinction when they consent to pass into the conquering nature, and
fire becomes air and air water. But if bodies of another kind go and
attack them [i.e. the small particles], the latter continue to be
dissolved until, being completely forced back and dispersed, they make
their escape to their own kindred, or else, being overcome and
assimilated to the conquering power, they remain where they are and
dwell with their victors, and from being many become one. And owing to
these affections, all things are changing their place, for by the
motion of the receiving vessel the bulk of each class is distributed
into its proper place; but those things which become unlike themselves
and like other things, are hurried by the shaking into the place of
the things to which they grow like.
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