Obj. 3: Further, as stated above (A. 5, Obj. 3), good is more distant
from evil, than venial from mortal sin. But an act which is evil in
itself, can become good; thus to kill a man may be an act of justice,
as when a judge condemns a thief to death. Much more therefore can a
mortal sin become venial.

_On the contrary,_ An eternal thing can never become temporal. But
mortal sin deserves eternal punishment, whereas venial sin deserves
temporal punishment. Therefore a mortal sin can never become venial.

_I answer that,_ Venial and mortal differ as perfect and imperfect in
the genus of sin, as stated above (A. 1, ad 1). Now the imperfect can
become perfect, by some sort of addition: and, consequently, a venial
sin can become mortal, by the addition of some deformity pertaining
to the genus of mortal sin, as when a man utters an idle word for the
purpose of fornication. On the other hand, the perfect cannot become
imperfect, by addition; and so a mortal sin cannot become venial, by
the addition of a deformity pertaining to the genus of venial sin,
for the sin is not diminished if a man commit fornication in order to
utter an idle word; rather is it aggravated by the additional
deformity.

Nevertheless a sin which is generically mortal, can become venial by
reason of the imperfection of the act, because then it does not
completely fulfil the conditions of a moral act, since it is not a
deliberate, but a sudden act, as is evident from what we have said
above (A. 2). This happens by a kind of subtraction, namely, of
deliberate reason. And since a moral act takes its species from
deliberate reason, the result is that by such a subtraction the
species of the act is destroyed.

Reply Obj. 1: Venial differs from mortal as imperfect from
perfect, even as a boy differs from a man. But the boy becomes a man
and not vice versa. Hence the argument does not prove.

Reply Obj. 2: If the ignorance be such as to excuse sin
altogether, as the ignorance of a madman or an imbecile, then he that
commits fornication in a state of such ignorance, commits no sin
either mortal or venial. But if the ignorance be not invincible, then
the ignorance itself is a sin, and contains within itself the lack of
the love of God, in so far as a man neglects to learn those things
whereby he can safeguard himself in the love of God.

Reply Obj. 3: As Augustine says (Contra Mendacium vii), "those
things which are evil in themselves, cannot be well done