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.
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 (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.
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.
I–II · Q. 74, Art. 5: Whether sin can be in the reason?
Whether sin can be in the reason?
Objection 1: It would seem that sin cannot be in the reason. For the sin of any power is a defect thereof. But the fault of the reason is not a sin, on the contrary, it excuses sin: for a man is excused from sin on account of ignorance. Therefore sin cannot be in the reason.
Objection 2: Further, the primary object of sin is the will, as stated above (A 1). Now reason precedes the will, since it directs it. Therefore sin cannot be in the reason.
Objection 3: Further, there can be no sin except about things which are under our control. Now perfection and defect of reason are not among those things which are under our control: since by nature some are mentally deficient, and some shrewd-minded. Therefore no sin is in the reason.
On the contrary, Augustine says (De Trin. xii, 12) that sin is in the lower and in the higher reason.
I answer that, The sin of any power is an act of that power, as we have clearly shown (AA 1,2,3). Now reason has a twofold act: one is its proper act in respect of its proper object, and this is the act of knowing the truth; the other is the act of reason as directing the other powers. Now in both of these ways there may be sin in the reason. First, in so far as it errs in the knowledge of truth, which error is imputed to the reason as a sin, when it is in ignorance or error about what it is able and ought to know: secondly, when it either commands the inordinate movements of the lower powers, or deliberately fails to check them.
Reply to Objection 1: This argument considers the defect in the proper act of the reason in respect of its proper object, and with regard to the case when it is a defect of knowledge about something which one is unable to know: for then this defect of reason is not a sin, and excuses from sin, as is evident with regard to the actions of madmen. If, however, the defect of reason be about something which a man is able and ought to know, he is not altogether excused from sin, and the defect is imputed to him as a sin. The defect which belongs only to the act of directing the other powers, is always imputed to reason as a sin, because it can always obviate this defect by means of its proper act.
Reply to Objection 2: As stated above (Q 17, A 1), when we were treating of the acts of the will and reason, the will moves and precedes the reason, in one way, and the reason moves and precedes the will in another: so that both the movement of the will can be called rational, and the act of the reason, voluntary. Accordingly sin is found in the reason, either through being a voluntary defect of the reason, or through the reason being the principle of the will's act.
The Reply to the Third Objection is evident from what has been said (ad 1).