timaeus   

the earth unmelted and undissolved; but particles of water, which
are larger, force a passage, and dissolve and melt the earth.
Wherefore earth when not consolidated by force is dissolved by water
only; when consolidated, by nothing but fire; for this is the only
body which can find an entrance. The cohesion of water again, when
very strong, is dissolved by fire only-when weaker, then either by air
or fire-the former entering the interstices, and the latter
penetrating even the triangles. But nothing can dissolve air, when
strongly condensed, which does not reach the elements or triangles; or
if not strongly condensed, then only fire can dissolve it. As to
bodies composed of earth and water, while the water occupies the
vacant interstices of the earth in them which are compressed by force,
the particles of water which approach them from without, finding no
entrance, flow around the entire mass and leave it undissolved; but
the particles of fire, entering into the interstices of the water,
do to the water what water does to earth and fire to air, and are
the sole causes of the compound body of earth and water liquefying and
becoming fluid. Now these bodies are of two kinds; some of them,
such as glass and the fusible sort of stones, have less water than
they have earth; on the other hand, substances of the nature of wax
and incense have more of water entering into their composition.
I have thus shown the various classes of bodies as they are
diversified by their forms and combinations and changes into one
another, and now I must endeavour to set forth their affections and
the causes of them. In the first place, the bodies which I have been
describing are necessarily objects of sense. But we have not yet
considered the origin of flesh, or what belongs to flesh, or of that
part of the soul which is mortal. And these things cannot be
adequately explained without also explaining the affections which
are concerned with sensation, nor the latter without the former: and
yet to explain them together is hardly possible; for which reason we
must assume first one or the other and afterwards examine the nature
of our hypothesis. In order, then, that the affections may follow
regularly after the elements, let us presuppose the existence of
body and soul.
First, let us enquire what we mean by saying that fire is hot; and
about this we may reason from the dividing or cutting power which it
exercises on our bodies. We all of us feel that fire is sharp; and
we may further consider the fineness of the sides, and the sharpness
of the angles, and the smallness of the particles, and the swiftness
of the motion-all this makes the action of fire violent and sharp,
so that it cuts whatever it meets. And we must not forget that the
original figure of fire [i.e. the pyramid], more than any other
form, has a dividing power which cuts our bodies into small pieces
(Kepmatizei), and thus naturally produces that affection which we call
heat; and hence the origin of the name (thepmos, Kepma). Now, the
opposite of this is sufficiently manifest; nevertheless we will not
fail to describe it. For the larger particles of moisture which
surround the body, entering in and driving out the lesser, but not
being able to take their places, compress the moist principle in us;
and this from being unequal and disturbed, is forced by them into a
state of rest, which is due to equability and compression. But
things which are contracted contrary to nature are by nature at war,
and force themselves apart; and to this war and convulsion the name of
shivering and trembling is given; and the whole affection and the
cause of the affection are both termed cold. That is called hard to
which our flesh yields, and soft which yields to our flesh; and things
are also termed hard and soft relatively to one another. That which
yields has a small base; but that which rests on quadrangular bases is
firmly posed and belongs to the class which offers the greatest
resistance; so too does that which is the most compact and therefore
most repellent. The nature of the light and the heavy will be best
understood when examined in connexion with our notions of above and
below; for it is quite a mistake to suppose that the universe is