Objection 1. It would seem that sins of commission and omission differ specifically. For "offense" and "sin" are condivided with one another (Ephesians 2:1), where it is written: "When you were dead in your offenses and sins," which words a gloss explains, saying: "'Offenses,' by omitting to do what was commanded, and 'sins,' by doing what was forbidden." Whence it is evident that "offenses" here denotes sins of omission; while "sin" denotes sins of commission. Therefore they differ specifically, since they are contrasted with one another as different species.
Objection 2. Further, it is essential to sin to be against God's law, for this is part of its definition, as is clear from what has been said (71, 6). Now in God's law, the affirmative precepts, against which is the sin of omission, are different from the negative precepts, against which is the sin of omission. Therefore sins of omission and commission differ specifically.
Objection 3. Further, omission and commission differ as affirmation and negation. Now affirmation and negation cannot be in the same species, since negation has no species; for "there is neither species nor difference of non-being," as the Philosopher states (Phys. iv, text. 67). Therefore omission and commission cannot belong to the same species.
On the contrary, Omission and commission are found in the same species of sin. For the covetous man both takes what belongs to others, which is a sin of commission; and gives not of his own to whom he should give, which is a sin of omission. Therefore omission and commission do not differ specifically.
I answer that, There is a twofold difference in sins; a material difference and a formal difference: the material difference is to be observed in the natural species of the sinful act; while the formal difference is gathered from their relation to one proper end, which is also their proper object. Hence we find certain acts differing from one another in the material specific difference, which are nevertheless formally in the same species of sin, because they are directed to the one same end: thus strangling, stoning, and stabbing come under the one species of murder, although the actions themselves differ specifically according to the natural species. Accordingly, if we refer to the material species in sins of omission and commission, they differ specifically, using species in a broad sense, in so far as negation and privation may have a species. But if we refer to the formal species of sins of omission and commission, they do not differ specifically, because they are directed to the same end, and proceed from the same motive. For the covetous man, in order to hoard money, both robs, and omits to give what he ought, and in like manner, the glutton, to satiate his appetite, both eats too much and omits the prescribed fasts. The same applies to other sins: for in things, negation is always founded on affirmation, which, in a manner, is its cause. Hence in the physical order it comes under the same head, that fire gives forth heat, and that it does not give forth cold.
Reply to Objection 1. This division in respect of commission and omission, is not according to different formal species, but only according to material species, as stated.
Reply to Objection 2. In God's law, the necessity for various affirmative and negative precepts, was that men might be gradually led to virtue, first by abstaining from evil, being induced to this by the negative precepts, and afterwards by doing good, to which we are induced by the affirmative precepts. Wherefore the affirmative and negative precepts do not belong to different virtues, but to different degrees of virtue; and consequently they are not of necessity, opposed to sins of different species. Moreover sin is not specified by that from which it turns away, because in this respect it is a negation or privation, but by that to which it turns, in so far as sin is an act. Consequently sins do not differ specifically according to the various precepts of the Law.
Reply to Objection 3. This objection considers the material diversity of sins. It must be observed, however, that although, properly speaking, negation is not in a species, yet it is allotted to a species by reduction to the affirmation on which it is based.
Source: Summa Theologica (New Advent)