inordinate movement may arise about
this also: thus when a man, in order to avoid the movements of
concupiscence, turns his thoughts away from carnal pleasures, to the
considerations of science, sometimes an unpremeditated movement of
vainglory will arise. Consequently, a man cannot avoid all such
movements, on account of the aforesaid corruption: but it is enough,
for the conditions of a voluntary sin, that he be able to avoid each
single one.

Reply Obj. 3: Man does not do perfectly himself what he does without
the deliberation of reason, since the principal part of man does
nothing therein: wherefore such is not perfectly a human act; and
consequently it cannot be a perfect act of virtue or of sin, but is
something imperfect of that kind. Therefore such movement of the
sensuality as forestalls the reason, is a venial sin, which is
something imperfect in the genus of sin.
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FOURTH ARTICLE [I-II, Q. 74, Art. 4]

Whether Mortal Sin Can Be in the Sensuality?

Objection 1: It would seem that mortal sin can be in the sensuality.
Because an act is discerned by its object. Now it is possible to
commit a mortal sin about the objects of the sensuality, e.g. about
carnal pleasures. Therefore the act of the sensuality can be a mortal
sin, so that mortal sin can be found in the sensuality.

Obj. 2: Further, mortal sin is opposed to virtue. But virtue can be
in the sensuality; for temperance and fortitude are virtues of the
irrational parts, as the Philosopher states (Ethic. iii, 10).
Therefore, since it is natural to contraries to be about the same
subject, sensuality can be the subject of mortal sin.

Obj. 3: Further, venial sin is a disposition to mortal sin. Now
disposition and habit are in the same subject. Since therefore venial
sin may be in the sensuality, as stated above (A. 3, ad 3), mortal
sin can be there also.

_On the contrary,_ Augustine says (Retract. i, 23): "The inordinate
movement of concupiscence, which is the sin of the sensuality, can
even be in those who are in a state of grace," in whom, however,
mortal sin is not to be found. Therefore the inordinate movement of
the sensuality is not a mortal sin.

_I answer that,_ Just as a disorder which destroys the principle of
the body's life causes the body's death, so too a disorder which
destroys the principle of spiritual life, viz. the last end, causes
spiritual death, which is mortal sin, as stated above (Q. 72, A. 5).
Now it