Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I-II, q. 71, a. 6; q. 74, a. 2; de Malo, q. 2, a. 2 and 3.

422. What is an external sin of commission. — A sin of commission properly so called is not only the interior will of evil, but also the external evil work commanded by the will or not prevented by it, e.g. homicide, theft, an external movement of anger: for although an external act does not have the character of fault except from the will, yet of itself it has the character of evil because it has a deformity or incongruity with right reason and the law of God, on account of undue matter or undue circumstances. Whence it is also prohibited by God: but whatever is prohibited by God is sin (and therefore to be declared in confession).

External sin is begun interiorly in the affection when its cause is voluntarily placed, and is consummated exteriorly in the effect when the external act is really present, in the same way as was said about omission.

423. External sin can be voluntary in cause or only indirectly. — For sin is an evil human act; therefore, just as for a human act, so for an external evil work to be sin, it is required and also suffices that it be voluntary in some way. Therefore it is not required that it be voluntary in itself, but it suffices that it be voluntary in a positive cause (as e.g. anger from drunkenness), or indirectly in a preceding omission inasmuch as the will ought to prevent it and does not (as e.g. damage from the fact that a father does not prevent his son from stealing).

424. Question: Is actual advertence to malice required for sin? — It is answered, from what has been said the question is easily solved: For sin some voluntariness suffices, whether in cause or indirectly from omission; but for such voluntariness, permittance at the moment when such cause or omission is placed suffices. Hence:

a) Actual advertence to malice is not always required here and now, i.e. at the moment when the sin is exteriorly consummated or the external act is effectively placed: for this is not required in order for it to be voluntary, for evil effects or acts placed with culpable inadvertence and proceeding from a voluntary cause from which they are foreseen to follow, are rightly imputed to man, as from culpable ignorance through negligence (e.g. harm to a patient from a physician negligent to learn what he is bound to know), from a passion which one deliberately wills to follow (e.g. homicide from drunkenness or anger), from an evil habit (e.g. blasphemy from custom), from voluntary inconsideration as by working precipitately (e.g. a hunter negligent to consider whether a man or a beast is present and killing a man).

b) Nevertheless that inadvertence and the act placed with it will not be voluntary and culpable unless at the moment when the cause was placed actual advertence was present by which the effect or evil act was foreseen at least confusedly, obscurely, and indistinctly, inasmuch as one at least doubted or suspected that evil would follow, or knew the obligation to attend to it or to use greater caution; otherwise in no way would they be voluntary. At this moment the sin is interiorly begun and committed as to affection and guilt.

From the given explanation the opposing opinions of theologians are easily reconciled, some of whom always require actual advertence, others on the contrary affirm that a virtual and interpretative [advertence] always suffices (n. 60, II, and 63).