summa-contra-gentiles

CHAPTER XXX
is not contrary to the natural disposition of the movable
subject, wherefore it is not a violent but a natural movement.
It is the same in the alteration of lower bodies by
the heavenly bodies : for there is a natural inclination in
the lower bodies to receive the influence of the higher
bodies. It is also thus in the generation of the elements :
since the form to be introduced by generation is not contrary
to primary matter, which is the subject of generation,
although it is contrary to the form to be cast aside, because
matter under a contrary form is not the subject of generation.
Accordingly
it is clear from what we have said that
the necessity resulting from an efficient cause depends, in
some things, on the disposition of the agent alone, but in
others on the disposition of both agent and patient. If then
this disposition, by reason of which the effect follows of
necessity, be absolutely necessary in both agent and patient,
there will be absolute necessity in the efficient cause : as in
those things which act necessarily and always. On the
other hand, if it be not absolutely necessary but may be
removed, no necessity will result from the efficient cause
except on the supposition that both have the disposition
required for action : as, for instance, in those things which
are sometimes hindered in their operation either through
defective power, or through the violence of a contrary :
wherefore they do not act always and necessarily, but in
the majority of cases.
From a final cause there results necessity in things in
two ways. In one way, forasmuch as it is first in the
intention of the agent. In this respect necessity results
from the end in the same way as from the agent since the
:
agent acts in so far as it intends the end, both in natural and
in voluntary actions. For in natural things, the intention
of the end belongs to the agent according to the latter's
form, whereby the end is becoming to it : wherefore the
natural thing must needs tend to the end according to
the virtue of its form : thus a heavy body tends towards the
centre according to the measure of its gravity. And in
voluntary matters, the will inclines to act for the sake of an
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