Summa summae S. Thomae, Tomus II

It is certain 1° that these sins, considered materially and in physical being, differ not only in species, but in genus ¹. For the sin of commission, materially considered, is a positive act deprived of rectitude; the sin of omission is the lack or privation of an act owed. But of act and non-act, of being and non-being, there is no univocal genus, that is, expressed by one and the same name in the same sense.

It is certain 2° that sins of commission and omission, even considered formally and in moral being, can be distinguished in species, if they are exercised about diverse matters; for it is evident that omission of hearing Mass is distinguished in moral species from theft.

But the question is whether, if they are referred to the same object and proceed from the same motive, they are distinguished in species, as theft and non-restoration of a theft. Wherefore

I say: Sins of omission and commission, considered in moral being, if they proceed about the same object from the same motive, are of the same species ².

It is proved. 1º. Sins, like other human acts, have their species from the end. Therefore, if the sin of commission and of omission are ordered to the same end, or proceed from the same motive, they will be of the same species: namely, if they are ordered to the same end of the work, they will be of the same essential species, as to seize another's goods and not restore them; if to the same end of the agent, they will be of the same accidental species, as to omit Mass and steal in order to commit adultery.

It is proved. 2º. To do good and, from the same motive, to omit the opposite evil do not constitute distinct moral species, but are integral parts of the same justice, as St. Thomas teaches, II II°, q. 79, a. 1, c, according to that of Psalm XXXI, 15: Decline from evil and do good, « because both, says the holy Doctor, are required for the perfect act of justice. » But proportionally they stand thus: in the genus of evil, to do evil and, from the same motive, to omit the opposite good, stand as, in the genus of good, to do good and to omit evil. Therefore.

It is confirmed by the example of motion. Although in motion recession from the term from which and access to the term to which involve two changes, namely from being to non-being, and from non-being to being, they nevertheless do not constitute diverse species of motion, but one, which is adequately constituted from both as from integral parts. But in the motion of sin, recession from good takes place through omission, access to evil through commission. Therefore.

From these you will rightly observe, according to the mind of St. Thomas, that sins of commission and omission are not said to be of the same species as two individuals, e.g., two men contracting the same human species, but because they stand as integral parts from which one species in the genus of morals is adequately constituted, in that proportional manner in which the integral parts of penance constitute one sacrament.

From what has been said you will infer that an act which is borne directly upon an omission, e.g.: « I will not hear Mass, » although it be a commission, belongs nevertheless to the sin of omission, and is of the same moral species with it.

You will ask which sin is the graver, whether of commission or of omission.

I answer. Speaking per se and other things being equal, the sin of commission is the graver. For it is graver to hate God than, under pressure of a precept, not to love him; graver to afflict parents with insult than not to reverence them at the proper time.