Thomas Aquinas

“ Omission, as stated above, is only of such good as is due and to which one is bound. Now no man is bound to the impossible: wherefore no man sins by omission, if he does not do what he cannot. Accordingly she who is violated after vowing virginity, is guilty of an omission, not through not having virginity, but through not repenting of her past sin, or through not doing what she can to fulfil her vow by observing continence. Again a priest is not bound to say Mass, except he have a suitable opportunity, and if this be lacking, there is no omission. ”

“ Wherefore a thing is further removed from its contrary than from its simple negation; thus black is further removed from white than not-white is, since every black is not-white, but not conversely. Now it is evident that transgression is contrary to an act of virtue, while omission denotes the negation thereof: for instance it is a sin of omission, if one fail to give one's parents due reverence, while it is a sin of transgression to revile them or injure them in any way. ”

“ And in this sense it belongs to special justice to do good considered as due to one's neighbor, and to avoid the opposite evil, that, namely, which is hurtful to one's neighbor; while it belongs to general justice to do good in relation to the community or in relation to God, and to avoid the opposite evil. ”

“ Omission is not original but actual sin, not as though it had some act essential to it, but for as much as the negation of an act is reduced to the genus of act, and in this sense non-action is a kind of action, as stated above (FS, Q [71] , A [6] , ad 1) . ”