be capable of sin. Wherefore it is not possible for this good of
nature to be destroyed entirely.

Since, however, this same good of nature may be continually
diminished by sin, some, in order to illustrate this, have made use
of the example of a finite thing being diminished indefinitely,
without being entirely destroyed. For the Philosopher says (Phys. i,
text. 37) that if from a finite magnitude a continual subtraction be
made in the same quantity, it will at last be entirely destroyed, for
instance if from any finite length I continue to subtract the length
of a span. If, however, the subtraction be made each time in the same
proportion, and not in the same quantity, it may go on indefinitely,
as, for instance, if a quantity be halved, and one half be diminished
by half, it will be possible to go on thus indefinitely, provided
that what is subtracted in each case be less than what was subtracted
before. But this does not apply to the question at issue, since a
subsequent sin does not diminish the good of nature less than a
previous sin, but perhaps more, if it be a more grievous sin.

We must, therefore, explain the matter otherwise by saying that the
aforesaid inclination is to be considered as a middle term between
two others: for it is based on the rational nature as on its root,
and tends to the good of virtue, as to its term and end. Consequently
its diminution may be understood in two ways: first, on the part of
its root, secondly, on the part of its term. In the first way, it is
not diminished by sin, because sin does not diminish nature, as
stated above (A. 1). But it is diminished in the second way, in so
far as an obstacle is placed against its attaining its term. Now if
it were diminished in the first way, it would needs be entirely
destroyed at last by the rational nature being entirely destroyed.
Since, however, it is diminished on the part of the obstacle which is
placed against its attaining its term, it is evident that it can be
diminished indefinitely, because obstacles can be placed
indefinitely, inasmuch as man can go on indefinitely adding sin to
sin: and yet it cannot be destroyed entirely, because the root of
this inclination always remains. An example of this may be seen in a
transparent body, which has an inclination to receive light, from the
very fact that it is transparent; yet this inclination or aptitude is
diminished on the part of supervening clouds, although it always
remains rooted