neither does interpretative. Best of all is actual attention, but it is
not necessary, because it is not always within the power of mortals.

This want of internal attention is called mental distraction.
Theologians distinguish two kinds of distractions, voluntary and
involuntary. Voluntary distractions are thoughts which the mind freely
and directly embraces to the exclusion of pious thoughts which should
occupy it in prayer, of which the office is a high form; or they may be
thoughts which arise from previous laziness, thoughtlessness,
pre-occupation or some engrossing worldly affair. Involuntary
distractions are those which come unbidden and unsought to the mind, are
neither placed directly, nor by their causes, by the person at prayer.

Does a person reciting the Hours sin if he have distractions?

If the distractions be involuntary there is no sin. But if the
distractions be voluntary there is sin, But, unless the mind be
altogether filled with distractions, not thinking of God, of prayer, of
the words or of the meaning, and unless the distractions are _fully
voluntary_ and _reflective_ during a notable part of the office, there
is no mortal sin. Hence, St. Alphonsus, the great Doctor of Prayer,
wrote, "_ut dicatur aliquis officio non satisfacere, non solum
requiritur ut voluntarie se distrahat, sed etiam ut plene advertat se
distrahi, nam alias iste, licet sponte se divertat non tamen sponte se
divertit a recitatione_" (St. Alphonsus, n. 177). Therefore, before a
person accuse himself of not satisfying the precept of recitation, on
account of inattention or distractions, he must be able to affirm
positively (1)that he was wilfully distracted, (2)he must have noticed
not only his distraction and mental occupation by vain thoughts, but he
must have noticed _also_ that he was distracted in his recitation; (3)he