Hence he that sins in such matters, for instance, by heresy, sacrilege, or blasphemy, is said to sin against God.  In like manner, the first order includes the third and surpasses it, because in all things wherein we are directed in reference to our neighbor, we need to be directed according to the order of reason.  Yet in some things we are directed according to reason, in relation to ourselves only, and not in reference to our neighbor; and when man sins in these matters, he is said to sin against himself, as is seen in the glutton, the lustful, and the prodigal.  But when man sins in matters concerning his neighbor, he is said to sin against his neighbor, as appears in the thief and murderer.  Now the things whereby man is directed to God, his neighbor, and himself are diverse.  Wherefore this distinction of sins is in respect of their objects, according to which the species of sins are diversified:  and consequently this distinction of sins is properly one of different species of sins:  because the virtues also, to which sins are opposed, differ specifically in respect of these three.  For it is evident from what has been said (Q. 62, AA. 1, 2, 3) that by the theological virtues man is directed to God; by temperance and fortitude, to himself; and by justice to his neighbor.

Reply Obj. 1:  To sin against God is common to all sins, in so far as the order to God includes every human order; but in so far as order to God surpasses the other two orders, sin against God is a special kind of sin.

Reply Obj. 2:  When several things, of which one includes another, are distinct from one another, this distinction is understood to refer, not to the part contained in another, but to that in which one goes beyond another.  This may be seen in the division of numbers and figures:  for a triangle is distinguished from a four-sided figure not in respect of its being contained thereby, but in respect of that in which it is surpassed thereby:  and the same applies to the numbers three and four.

Reply Obj. 3:  Although God and our neighbor are external to the sinner himself, they are not external to the act of sin, but are related to it as to its object. ________________________

FIFTH ARTICLE [I-II, Q. 72, Art. 5]

Whether the Division of Sins According to Their Debt of Punishment
Diversifies Their Species?

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 Deut. 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.