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.
