Summa Theologiae Moralis, vol. I: De Principiis

S. Th., I-II, q. 71, a. 5; De Malo, q. 2, a. 1.

419. What is a sin of omission? — A sin of commission is the placement of a forbidden act, and is opposed to a negative precept; a sin of omission is the non-placement of a prescribed act, and is opposed to an affirmative precept.

This sin of omission in itself, i.e. considered in its essence, is not an act but a mere privation; yet it supposes some positive act, at least interior, conjoined or preceding, which is its cause or occasion, by which namely one directly wills to omit the prescribed thing, or directly wills, whether now or previously, something else by which he is hindered from doing what he ought. For omission, to be a sin, must be directly voluntary, either in itself, as if one wills not to hear Mass, or at least in cause, as if one directly wills to place an act incompatible with the prescribed thing, whether conjoined, as e.g. if one wills to play when he ought to hear Mass, or preceding, as if one wills to stay up late at night, from which he foresees that he will not hear it¹.

420. When is it a sin of omission? — A sin of omission is begun interiorly and contracted as to affection and guilt when the cause of the omission is voluntarily placed: for then the one omitting either voluntarily intends not to fulfill, or voluntarily exposes himself to the danger of not fulfilling; it is consummated exteriorly and placed effectively when the time of fulfilling the precept is present and it is not fulfilled.

This exterior omission adds no malice of itself to the interior act; nevertheless it is sin (and therefore to be declared in confession) as being a voluntary inordination, in the same way as an external evil work is rightly called sin. But if one has sincerely and efficaciously withdrawn the will to omit before the exterior omission follows, this will not be sin but only the effect of sin materially evil, as e.g. one who threw the breviary into the sea, and afterward repents on the journey.

421. Is a work placed at the time of omission a sin? — A work placed at the time of omission, if it is the cause or occasion of it, is a sin, yet not distinct from the omission (and accordingly not to be declared in confession), but evil by the very malice of the omission: thus a game which is the cause of the omission of Mass. If it is not the cause of the omission but is merely accidentally and concomitantly related to the omission, unless it is evil from elsewhere, it will not be evil nor from the omission: thus if one has efficaciously resolved not to hear Mass, but, lest he meanwhile remain idle, applies himself to study.