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 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.
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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