at sin, since voluntariness is
essential to sin. Sometimes, however, the act of the will bears
directly on something else which hinders man from doing what he
ought, whether this something else be united with the omission, as
when a man wills to play at the time he ought to go to church--or,
precede the omission, as when a man wills to sit up late at night,
the result being that he does not go to church in the morning. In
this case the act, interior or exterior, is accidental to the
omission, since the omission follows outside the intention, and that
which is outside the intention is said to be accidental (Phys. ii,
text. 49, 50). Wherefore it is evident that then the sin of omission
has indeed an act united with, or preceding the omission, but that
this act is accidental to the sin of omission.

Now in judging about things, we must be guided by that which is
proper to them, and not by that which is accidental: and consequently
it is truer to say that a sin can be without any act; else the
circumstantial acts and occasions would be essential to other actual
sins.

Reply Obj. 1: More things are required for good than for evil, since
"good results from a whole and entire cause, whereas evil results
from each single defect," as Dionysius states (Div. Nom. iv): so that
sin may arise from a man doing what he ought not, or by his not doing
what he ought; while there can be no merit, unless a man do willingly
what he ought to do: wherefore there can be no merit without act,
whereas there can be sin without act.

Reply Obj. 2: The term "voluntary" is applied not only to that on
which the act of the will is brought to bear, but also to that which
we have the power to do or not to do, as stated in _Ethic._ iii, 5.
Hence even not to will may be called voluntary, in so far as man has
it in his power to will, and not to will.

Reply Obj. 3: The sin of omission is contrary to an affirmative
precept which binds always, but not for always. Hence, by omitting to
act, a man sins only for the time at which the affirmative precept
binds him to act.
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SIXTH ARTICLE [I-II, Q. 71, Art. 6]

Whether Sin Is Fittingly Defined As a Word, Deed, or Desire Contrary
to the Eternal Law?

Objection 1: It would seem that sin is unfittingly defined by saying:
"Sin is a word, deed, or desire, contrary to the eternal law."
Because "word," "deed," and "desire" imply an act; whereas not every
sin implies an act, as stated ab