I answer that, Evil is more comprehensive than sin, as also is good than right.  For every privation of good, in whatever subject, is an evil:  whereas sin consists properly in an action done for a certain end, and lacking due order to that end.  Now the due order to an end is measured by some rule.  In things that act according to nature, this rule is the natural force that inclines them to that end.  When therefore an action proceeds from a natural force, in accord with the natural inclination to an end, then the action is said to be right:  since the mean does not exceed its limits, viz. the action does not swerve from the order of its active principle to the end.  But when an action strays from this rectitude, it comes under the notion of sin.

Now in those things that are done by the will, the proximate rule is the human reason, while the supreme rule is the Eternal Law.  When, therefore, a human action tends to the end, according to the order of reason and of the Eternal Law, then that action is right:  but when it turns aside from that rectitude, then it is said to be a sin.  Now it is evident from what has been said (Q. 19, AA. 3, 4) that every voluntary action that turns aside from the order of reason and of the Eternal Law, is evil, and that every good action is in accord with reason and the Eternal Law.  Hence it follows that a human action is right or sinful by reason of its being good or evil.

Reply Obj. 1:  Monsters are called sins, inasmuch as they result from a sin in nature’s action.

Reply Obj. 2:  The end is twofold; the last end, and the proximate end.  In the sin of nature, the action does indeed fail in respect of the last end, which is the perfection of the thing generated; but it does not fail in respect of any proximate end whatever; since when nature works it forms something.  In like manner, the sin of the will always fails as regards the last end intended, because no voluntary evil action can be ordained to happiness, which is the last end:  and yet it does not fail in respect of some proximate end:  intended and achieved by the will.  Wherefore also, since the very intention of this end is ordained to the last end, this same intention may be right or sinful.

Reply Obj. 3:  Each thing is ordained to its end by its action:  and therefore sin, which consists in straying from the order to the end, consists properly in an action.  On the other hand, punishment regards the person of the sinner, as was stated in the First Part (Q. 48, A. 5, ad 4; A. 6, ad 3). ________________________

SECOND ARTICLE [I-II, Q. 21, Art. 2]

Whether a Human Action Deserves Praise or Blame, by Reason of Its
Being Good or Evil?

Objection 1:  It would seem that a human action does not deserve praise or blame by reason of its being good or evil.  For “sin happens even in things done by nature” (Phys. ii, 8).  And yet natural things are not deserving of praise or blame (Ethic. iii, 5).  Therefore a human action does not deserve blame, by reason of its being evil or sinful; and, consequently, neither does it deserve praise, by reason of its being good.