With regard to drunkenness we reply that it is a mortal sin by reason of its genus; for, that a man, without necessity, and through the mere lust of wine, make himself unable to use his reason, whereby he is directed to God and avoids committing many sins, is expressly contrary to virtue.  That it be a venial sin, is due some sort of ignorance or weakness, as when a man is ignorant of the strength of the wine, or of his own unfitness, so that he has no thought of getting drunk, for in that case the drunkenness is not imputed to him as a sin, but only the excessive drink.  If, however, he gets drunk frequently, this ignorance no longer avails as an excuse, for his will seems to choose to give way to drunkenness rather than to refrain from excess of wine:  wherefore the sin returns to its specific nature.

Reply Obj. 2:  Morose delectation is not a mortal sin except in those matters which are mortal sins generically.  In such matters, if the delectation be not morose, there is a venial sin through imperfection of the act, as we have said with regard to anger (ad 1):  because anger is said to be lasting, and delectation to be morose, on account of the approval of the deliberating reason.

Reply Obj. 3:  A circumstance does not make a good act to be evil, unless it constitute the species of a sin, as we have stated above (Q. 18, A. 5, ad 4). ________________________

SIXTH ARTICLE [I-II, Q. 88, Art. 6]

Whether a Mortal Sin Can Become Venial?

Objection 1:  It would seem that a mortal sin can become venial.  Because venial sin is equally distant from mortal, as mortal sin is from venial.  But a venial sin can become mortal, as stated above (A. 5).  Therefore also a mortal sin can become venial.

Obj. 2:  Further, venial and mortal sin are said to differ in this, that he who sins mortally loves a creature more than God, while he who sins venially loves the creature less than God.  Now it may happen that a person in committing a sin generically mortal, loves a creature less than God; for instance, if anyone being ignorant that simple fornication is a mortal sin, and contrary to the love of God, commits the sin of fornication, yet so as to be ready, for the love of God, to refrain from that sin if he knew that by committing it he was acting counter to the love of God.  Therefore his will be a venial sin; and accordingly a mortal sin can become venial.

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.