o
do" does not imply an act. Therefore sin can be without act.

_I answer that,_ The reason for urging this question has reference to
the sin of omission, about which there have been various opinions.
For some say that in every sin of omission there is some act, either
interior or exterior--interior, as when a man wills _not to go to
church,_ when he is bound to go--exterior, as when a man, at the very
hour that he is bound to go to church (or even before), occupies
himself in such a way that he is hindered from going. This seems, in
a way, to amount to the same as the first, for whoever wills one
thing that is incompatible with this other, wills, consequently, to
go without this other: unless, perchance, it does not occur to him,
that what he wishes to do, will hinder him from that which he is
bound to do, in which case he might be deemed guilty of negligence.
On the other hand, others say, that a sin of omission does not
necessarily suppose an act: for the mere fact of not doing what one
is bound to do is a sin.

Now each of these opinions has some truth in it. For if in the sin of
omission we look merely at that in which the essence of the sin
consists, the sin of omission will be sometimes with an interior act,
as when a man wills _not to go to church:_ while sometimes it will be
without any act at all, whether interior or exterior, as when a man,
at the time that he is bound to go to church, does not think of going
or not going to church.

If, however, in the sin of omission, we consider also the causes, or
occasions of the omission, then the sin of omission must of necessity
include some act. For there is no sin of omission, unless we omit
what we can do or not do: and that we turn aside so as not to do what
we can do or not do, must needs be due to some cause or occasion,
either united with the omission or preceding it. Now if this cause be
not in man's power, the omission will not be sinful, as when anyone
omits going to church on account of sickness: but if the cause or
occasion be subject to the will, the omission is sinful; and such
cause, in so far as it is voluntary, must needs always include some
act, at least the interior act of the will: which act sometimes bears
directly on the omission, as when a man wills _not to go to church,_
because it is too much trouble; and in this case this act, of its
very nature, belongs to the omission, because the volition of any sin
whatever, pertains, of itself, to th