ngs in the course of time, they are no longer
able to cut or assimilate the food which enters, but are themselves
easily divided by the bodies which come in from without. In this way
every animal is overcome and decays, and this affection is called old
age. And at last, when the bonds by which the triangles of the marrow
are united no longer hold, and are parted by the strain of existence,
they in turn loosen the bonds of 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 proceeds in a wrong order; they are then destroyed. For the
natural order is that the flesh and sinews should be made of blood, the
sine