A Summary of the Question II of DE MALO by Thomas Aquinas
First Article: Is an Act Involved in Every Sin?
There are two opinions about this matter. First, there is an act in every sin, even in sin of omission. And the second, there can be a sin even without an act of the will. And each opinion is true in some respect.
1. From the essence of sin
If we consider what is required for there to be sin, as belonging to the essence of sin, then an act is not required for a sin of omission; rather, the sin of omission consists of the very desistance from action. For example, if a doctor should administer medicine according to the rules of medical practice, and the sick person should fail to be cured, whether because the sick person has an incurable disease, or because the sick person does something adverse to the person's health, the doctor is indeed not at fault even though he fails to achieve the end. And conversely, if the doctor were to achieve the end but were to deviate from the rules of medical practice, we would nonetheless say that the doctor erred. And it is evident from this that it belongs more to the nature of fault to disregard a rule of action than even to fail to attain the end of the action. Therefore, it belongs intrinsically to the nature of fault, whether in nature or human skills or moral matters, to be contrary to a rule of action.
This rule of action includes particular negative and affirmative orders that are contained in the natural power of reason and in the divine law, which ought to govern our actions. And so as we impute acting as sin in that action is contrary to a negative order of the law, so also we impute non acting itself as sin in that non action is contrary to an affirmative order of the law.
Therefore, there can be a sin for which an act belonging to the essence of sin is not required. (The second opinion)
2. From the cause of sin
If we consider what is required for sin regarding the cause of sin, then an act needs to be required for any sin, even one of omission. There needs to be a cause for a person not doing what the person should do.
And if the cause was totally extrinsic, such an omission does not have the nature of sin. We only impute omission as sin when it has an intrinsic voluntary cause, not an intrinsic cause of any kind. For if an intrinsic involuntary cause, for example, fever, were to prevent a person from doing what the person should do, the argument would be the same as in the case of an extrinsic cause doing so. Therefore, for the omission of a duty to be a sin, it is required that a voluntary act cause the omission.
But the will indeed sometimes causes something intrinsically and sometimes accidentally. The will indeed causes intrinsically when it intentionally acts to produce such an effect (e.g., if one seeking to find treasure, should find some in the course of digging). The will causes accidentally when the effect is outside one's intention (e.g., if one wanting to dig a grave should find treasure). Therefore, a voluntary act is sometimes the intrinsic cause of omitting a duty but not in such a way that the will is directly borne to the omission. This is because nonbeing and evil are outside one's intention and one's power to will, and the object of the will is being and good. Rather, the will is indirectly borne to something positive when one foresees the resulting omission. For example, one wills to play games, knowing that doing so means not going to church, and we say that a thief, not shrinking from the deformity of injustice, wants money. And a voluntary act is sometimes the accidental cause of omitting a duty, as when a person occupied in some activity does not recall something that the person is obliged to do. And it makes no difference in this regard whether the voluntary act that intrinsically or accidentally causes the omission is simultaneous with the omission itself or even precedes it. Therefore, the first opinion is true in this respect, that a voluntary act needs to cause the omission of a duty for the omission to be sinful.