timaeus   

the soul, and she, obtaining a natural release, flies away with joy.
For that which takes place according to nature is pleasant, but that
which is contrary to nature is painful. And thus death, if caused by
disease or produced by wounds, is painful and violent; but that sort
of death which comes with old age and fulfils the debt of nature is
the easiest of deaths, and is accompanied with pleasure rather than
with pain.
Now every one can see whence diseases arise. There are four
natures out of which the body is compacted, earth and fire and water
and air, and the unnatural excess or defect of these, or the change of
any of them from its own natural place into another, or-since there
are more kinds than one of fire and of the other elements-the
assumption by any of these of a wrong kind, or any similar
irregularity, produces disorders and diseases; for when any of them is
produced or changed in a manner contrary to nature, the parts which
were previously cool grow warm, and those which were dry become moist,
and the light become heavy, and the heavy light; all sorts of
changes occur. For, as we affirm, a thing can only remain the same
with itself, whole and sound, when the same is added to it, or
subtracted from it, in the same respect and in the same manner and
in due proportion; and whatever comes or goes away in violation of
these laws causes all manner of changes and infinite diseases and
corruptions. Now there is a second class of structures which are
also natural, and this affords a second opportunity of observing
diseases to him who would understand them. For whereas marrow and bone
and flesh and sinews are composed of the four elements, and the blood,
though after another manner, is likewise formed out of them, most
diseases originate in the way which I have described; but the worst of
all owe their severity to the fact that the generation of these
substances stances in a wrong order; they are then destroyed. For
the natural order is that the flesh and sinews should be made of
blood, the sinews out of the fibres to which they are akin, and the
flesh out of the dots which are formed when the fibres are
separated. And the glutinous and rich matter which comes away from the
sinews and the flesh, not only glues the flesh to the bones, but
nourishes and imparts growth to the bone which surrounds the marrow;
and by reason of the solidity of the bones, that which filters through
consists of the purest and smoothest and oiliest sort of triangles,
dropping like dew from the bones and watering the marrow.
Now when each process takes place in this order, health commonly
results; when in the opposite order, disease. For when the flesh
becomes decomposed and sends back the wasting substance into the
veins, then an over-supply of blood of diverse kinds, mingling with
air in the veins, having variegated colours and bitter properties,
as well as acid and saline qualities, contains all sorts of bile and
serum and phlegm. For all things go the wrong way, and having become
corrupted, first they taint the blood itself, and then ceasing to give
nourishment the body they are carried along the veins in all
directions, no longer preserving the order of their natural courses,
but at war with themselves, because they receive no good from one
another, and are hostile to the abiding constitution of the body,
which they corrupt and dissolve. The oldest part of the flesh which is
corrupted, being hard to decompose, from long burning grows black, and
from being everywhere corroded becomes bitter, and is injurious to
every part of the body which is still uncorrupted. Sometimes, when the
bitter element is refined away, the black part assumes an acidity
which takes the place of the bitterness; at other times the bitterness
being tinged with blood has a redder colour; and this, when mixed with
black, takes the hue of grass; and again, an auburn colour mingles
with the bitter matter when new flesh is decomposed by the fire
which surrounds the internal flame-to all which symptoms some
physician perhaps, or rather some philosopher, who had the power of
seeing in many dissimilar things one nature deserving of a name, has
assigned the common name of bile. But the other kinds of bile are