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.

Obj. 2: Further, just as sin occurs in moral actions, so does it
happen in the productions of art: because as stated in _Phys._ ii, 8
"it is a sin in a grammarian to write badly, and in a doctor to give
the wrong medicine." But the artist is not blamed for making
something bad: because the artist's work is such, that he can produce
a good or