Reply Obj. 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 Obj. 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 Obj. 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. ________________________

FIFTH ARTICLE [I-II, Q. 74, Art. 5]

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.

Obj. 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.

Obj. 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.