vii, text. 27, a thing is a
cause of movement in two ways. First, directly; and such a thing
causes movement by reason of its proper form; thus fire causes heat.
Secondly, indirectly; for instance, that which removes an obstacle.
It is in this latter way that the destruction or diminution of a
habit results through cessation from act, in so far, to wit, as we
cease from exercising an act which overcame the causes that destroyed
or weakened that habit. For it has been stated (A. 1) that habits are
destroyed or diminished directly through some contrary agency.
Consequently all habits that are gradually undermined by contrary
agencies which need to be counteracted by acts proceeding from those
habits, are diminished or even destroyed altogether by long cessation
from act, as is clearly seen in the case both of science and of
virtue. For it is evident that a habit of moral virtue makes a man
ready to choose the mean in deeds and passions. And when a man fails
to make use of his virtuous habit in order to moderate his own
passions or deeds, the necessary result is that many passions and
deeds fail to observe the mode of virtue, by reason of the
inclination of the sensitive appetite and of other external agencies.
Wherefore virtue is destroyed or lessened through cessation from act.
The same applies to the intellectual habits, which render man ready
to judge aright of those things that are pictured by his imagination.
Hence when man ceases to make use of his intellectual habits, strange
fancies, sometimes in opposition to them, arise in his imagination;
so that unless those fancies be, as it were, cut off or kept back by
frequent use of his intellectual habits, man becomes less fit to
judge aright, and sometimes is even wholly disposed to the contrary,
and thus the intellectual habit is diminished or even wholly
destroyed by cessation from act.

Reply Obj. 1: Even heat would be destroyed through ceasing to give
heat, if, for this same reason, cold which is destructive of heat
were to increase.

Reply Obj. 2: Cessation from act is a moving cause, conducive of
corruption or diminution, by removing the obstacles thereto, as
explained above.

Reply Obj. 3: The intellectual part of the soul, considered in
itself, is above time, but the sensitive part is subject to time, and
therefore in course of time it undergoes change as to the passions of
the sensitive part, and also as to the powers of apprehension. Hence
the Philosopher