Reply Obj. 2:  The continual corruption of the sensuality is to be understood as referring to the fomes, which is never completely destroyed in this life, since, though the stain of original sin passes, its effect remains.  However, this corruption of the fomes does not hinder man from using his rational will to check individual inordinate movements, if he be presentient of them, for instance by turning his thoughts to other things.  Yet while he is turning his thoughts to something else, an 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. ________________________

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 belongs to the reason alone, and not to the sensuality, to order anything to the end:  and disorder in respect of the end can only belong to the power whose function it is to order others to the end.  Wherefore mortal sin cannot be in the sensuality, but only in the reason.