Consequently act surpasses
habit both in goodness and in badness.
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FOURTH ARTICLE [I-II, Q. 71, Art. 4]

Whether Sin Is Compatible with Virtue?

Objection 1: It would seem that a vicious act, i.e. sin, is
incompatible with virtue. For contraries cannot be together in the
same subject. Now sin is, in some way, contrary to virtue, as stated
above (A. 1). Therefore sin is incompatible with virtue.

Obj. 2: Further, sin is worse than vice, i.e. evil act than evil
habit. But vice cannot be in the same subject with virtue: neither,
therefore, can sin.

Obj. 3: Further, sin occurs in natural things, even as in voluntary
matters (Phys. ii, text. 82). Now sin never happens in natural
things, except through some corruption of the natural power; thus
monsters are due to corruption of some elemental force in the seed,
as stated in _Phys._ ii. Therefore no sin occurs in voluntary
matters, except through the corruption of some virtue in the soul: so
that sin and virtue cannot be together in the same subject.

_On the contrary,_ The Philosopher says (Ethic. ii, 2, 3) that
"virtue is engendered and corrupted by contrary causes." Now one
virtuous act does not cause a virtue, as stated above (Q. 51, A. 3):
and, consequently, one sinful act does not corrupt virtue. Therefore
they can be together in the same subject.

_I answer that,_ Sin is compared to virtue, as evil act to good habit.
Now the position of a habit in the soul is not the same as that of a
form in a natural thing. For the form of a natural thing produces, of
necessity, an operation befitting itself; wherefore a natural form is
incompatible with the act of a contrary form: thus heat is
incompatible with the act of cooling, and lightness with downward
movement (except perhaps violence be used by some extrinsic mover):
whereas the habit that resides in the soul, does not, of necessity,
produce its operation, but is used by man when he wills. Consequently
man, while possessing a habit, may either fail to use the habit, or
produce a contrary act; and so a man having a virtue may produce an
act of sin. And this sinful act, so long as there is but one, cannot
corrupt virtue, if we compare the act to the virtue itself as a habit:
since, just as habit is not engendered by one act, so neither is it
destroyed by one act as stated above (Q. 63, A. 2, ad 2). But if
we compare the sinful act to the cause of the virtues, then it is
possible for some vir