ffice commits
a mortal sin. A notable part of the Divine Office for any day is held by
some theologians to be the omission of one psalm in one of the small
hours, or a corresponding quantity of matter in lessons, responses, etc.
They hold that such wilful omission is a grave sin. Other theologians
hold--and their opinion is the more common and the more probable
one--that, although one psalm is a notable part of a small hour, in
relation to the whole office it is not a notable part, and its omission
is not a grave matter. These theologians hold that the wilful omission
of an entire small hour or equivalent matter (e.g., Sext, or the third
nocturn of Matins) is an omission of a notable part and cannot be
excused from grave sin.

The omission of the entire office of a day, the seven canonical hours,
is held by some theologians to carry the guilt of seven mortal sins.
Because, there is a different precept for each hour and the omission of
each hour violates a precept. The Salamenticenses think this opinion
probable. The more common and the more correct opinion is that by such
omission only one sin is committed. And the theologians who hold this
opinion say that the recitation of the canonical hours is imposed under
one precept only, and hence there is only one obligation embracing the
seven hours. This is the opinion of St. Alphonsus (n. 148) who quotes
several authors (including Lessius, Sanchez and St. Antoninus) in
support. If a person in Holy Orders omit several hours with a
retractation, or a moral interruption in his sinful intentions, he may
commit several mortal sins, because all the omissions, which in
themselves are grave matter, may become independent of each other by the
interruption and renewal of the intention (St. Alphonsus, n. 148).

What must a person do who has a doubt that he has omitted something in
his recitation of the office? Is he bound to make assurance doubly sure
by reciting the part of which he doubts?

If the doubt be a positive doubt, that is, if he have good reason to
believe that he has recited it, he is not bound to anything further
regarding the part in question. For instance, if a priest remembers
having started the recitation of a lesson, and in a short time finds
himself at the end of it, and cannot be sure if he have recited it, the
presumption is in favour of the priest and of the recitation, because it
is his custom to recite completely whatever part he commences. He has,
thus, mor