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.
Objection 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.
Objection 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 (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 (Question 72, Article 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.
Reply to Objection 1. The act of the sensuality can concur towards a mortal sin: yet the fact of its being a mortal sin is due, not to its being an act of the sensuality, but to its being an act of reason, to whom the ordering to the end belongs. Consequently mortal sin is imputed, not to the sensuality, but to reason.
Reply to Objection 2. An act of virtue is perfected not only in that it is an act of the sensuality, but still more in the fact of its being an act of reason and will, whose function it is to choose: for the act of moral virtue is not without the exercise of choice: wherefore the act of moral virtue, which perfects the appetitive power, is always accompanied by an act of prudence, which perfects the rational power; and the same applies to mortal sin, as stated (ad 1).
Reply to Objection 3. A disposition may be related in three ways to that to which it disposes: for sometimes it is the same thing and is in the same subject; thus inchoate science is a disposition to perfect science: sometimes it is in the same subject, but is not the same thing; thus heat is a disposition to the form of fire: sometimes it is neither the same thing, nor in the same subject, as in those things which are subordinate to one another in such a way that we can arrive at one through the other, e.g. goodness of the imagination is a disposition to science which is in the intellect. In this way the venial sin that is in the sensuality, may be a disposition to mortal sin, which is in the reason.
Source: Summa Theologica (New Advent)