ore 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 deserv