The failure to do something which one can and ought to do
Omission (Lat. omittere, to lay aside, to pass over) is here taken to be the failure to do something which one can and ought to do. If this happens advertently and freely a sin is committed. Moralists took pains formerly to show that the inaction implied in an omission was quite compatible with a breach of the moral law, for it is not merely because a person here and now does nothing that he offends, but because he neglects to act under circumstances in which he can and ought to act. The degree of guilt incurred by an omission is measured like that attaching to sins of commission, by the dignity of the virtue and the magnitude of the precept to which the omission is opposed as well as the amount of deliberation. In general, according to St. Thomas, the sin of omission consisting as it does in a leaving out of good is less grievous than a sin of commission which involves a positive taking up with evil. There are, of course, cases in which on account of the special subject matter and circumstances it may happen that an omission is more heinous. It may be asked at what time one incurs the guilt of a sin of omission in case he fails to do something which he is unable to do by reason of a cause for which he is entirely responsible. For instance, if a person fails to perform a duty in the morning as a result of becoming inebriated the previous night. The guilt is not incurred at the time the duty should be performed because while intoxicated he is incapable of moral guilt. The answer seems to be that he becomes responsible for the omission when having sufficiently foreseen that his neglect will follow upon his intoxication he does nevertheless surrender himself to his craving for liquor.
JOSEPH F. DELANY