Attention may be either external or internal. External attention is
attention of such a kind that it excludes every exterior action
physically incompatible with the recitation of the office--e.g., to
write or type a letter, to listen attentively to those conversing, are
acts incompatible with the simultaneous recitation of the office. But
walking, poking a fire, looking for the lessons, whilst reciting from
memory all the time, are not incompatible with the external attention
required in office recital; because such acts do not require mental
effort which could count as a serious disturbing element. However, in
this matter of external attention no rule can be formulated for all
Breviary readers; for what may lightly disturb and distract one reader
may have no effect on another, and yet may seriously disturb the
recitation of another (St. Alph., n. 176). External attention is
necessary for the valid recitation of the office.

Internal attention is application or advertence of the mind. Is such
internal attention, such deliberate application or mental advertence
necessary for the valid recitation of the office?

There are two opinions on this matter, two replies to the question.
According to one opinion, and this is the more common and the more
probable one, internal attention is required for the valid recitation of
the Hours. 1. Because the Divine Office is a prayer, but there can be no
true or real prayer without internal attention, for prayer is defined as
an elevation of the soul to God, but if there be no internal attention,
there is no elevation of the soul to God, and no prayer. 2. Our Lord
complained of those who had external attention at prayer, but lacked
internal attention or advertence, "This people honour me with their
lips, but their heart is far from me" (St. Matt. xv.). 3. The Church
appears to demand internal attention at prayer, for although she has not