Objection 1. It would seem that the division of sins according to their debt of punishment diversifies their species; for instance, when sin is divided into "mortal" and "venial." For things which are infinitely apart, cannot belong to the same species, nor even to the same genus. But venial and mortal sin are infinitely apart, since temporal punishment is due to venial sin, and eternal punishment to mortal sin; and the measure of the punishment corresponds to the gravity of the fault, according to Deuteronomy 25:2: "According to the measure of the sin shall the measure be also of the stripes be." Therefore venial and mortal sins are not of the same genus, nor can they be said to belong to the same species.
Objection 2. Further, some sins are mortal in virtue of their species ["Ex genere," genus in this case denoting the species], as murder and adultery; and some are venial in virtue of their species, as in an idle word, and excessive laughter. Therefore venial and mortal sins differ specifically.
Objection 3. Further, just as a virtuous act stands in relation to its reward, so does sin stand in relation to punishment. But the reward is the end of the virtuous act. Therefore punishment is the end of sin. Now sins differ specifically in relation to their ends, as stated above (1, ad 1). Therefore they are also specifically distinct according to the debt of punishment.
On the contrary, Those things that constitute a species are prior to the species, e.g. specific differences. But punishment follows sin as the effect thereof. Therefore sins do not differ specifically according to the debt of punishment.
I answer that, In things that differ specifically we find a twofold difference: the first causes the diversity of species, and is not to be found save in different species, e.g. "rational" and "irrational," "animate," and "inanimate": the other difference is consequent to specific diversity; and though, in some cases, it may be consequent to specific diversity, yet, in others, it may be found within the same species; thus "white" and "black" are consequent to the specific diversity of crow and swan, and yet this difference is found within the one species of man.
We must therefore say that the difference between venial and mortal sin, or any other difference is respect of the debt of punishment, cannot be a difference constituting specific diversity. For what is accidental never constitutes a species; and what is outside the agent's intention is accidental (Phys. ii, text. 50). Now it is evident that punishment is outside the intention of the sinner, wherefore it is accidentally referred to sin on the part of the sinner. Nevertheless it is referred to sin by an extrinsic principle, viz. the justice of the judge, who imposes various punishments according to the various manners of sin. Therefore the difference derived from the debt of punishment, may be consequent to the specific diversity of sins, but cannot constitute it.
Now the difference between venial and mortal sin is consequent to the diversity of that inordinateness which constitutes the notion of sin. For inordinateness is twofold, one that destroys the principle of order, and another which, without destroying the principle of order, implies inordinateness in the things which follow the principle: thus, in an animal's body, the frame may be so out of order that the vital principle is destroyed; this is the inordinateness of death; while, on the other hand, saving the vital principle, there may be disorder in the bodily humors; and then there is sickness. Now the principle of the entire moral order is the last end, which stands in the same relation to matters of action, as the indemonstrable principle does to matters of speculation (Ethic. vii, 8). Therefore when the soul is so disordered by sin as to turn away from its last end, viz. God, to Whom it is united by charity, there is mortal sin; but when it is disordered without turning away from God, there is venial sin. For even as in the body, the disorder of death which results from the destruction of the principle of life, is irreparable according to nature, while the disorder of sickness can be repaired by reason of the vital principle being preserved, so it is in matters concerning the soul. Because, in speculative matters, it is impossible to convince one who errs in the principles, whereas one who errs, but retains the principles, can be brought back to the truth by means of the principles. Likewise in practical matters, he who, by sinning, turns away from his last end, if we consider the nature of his sin, falls irreparably, and therefore is said to sin mortally and to deserve eternal punishment: whereas when a man sins without turning away from God, by the very nature of his sin, his disorder can be repaired, because the principle of the order is not destroyed; wherefore he is said to sin venially, because, to wit, he does not sin so as to deserve to be punished eternally.
Reply to Objection 1. Mortal and venial sins are infinitely apart as regards what they "turn away from," not as regards what they "turn to," viz. the object which specifies them. Hence nothing hinders the same species from including mortal and venial sins; for instance, in the species "adultery" the first movement is a venial sin; while an idle word, which is, generally speaking, venial, may even be a mortal sin.
Reply to Objection 2. From the fact that one sin is mortal by reason of its species, and another venial by reason of its species, it follows that this difference is consequent to the specific difference of sins, not that it is the cause thereof. And this difference may be found even in things of the same species, as stated above.
Reply to Objection 3. The reward is intended by him that merits or acts virtually; whereas the punishment is not intended by the sinner, but, on the contrary, is against his will. Hence the comparison fails.
Source: Summa Theologica (New Advent)