The valid recitation of the Divine Office requires that the priest should have in his mind an intention of praying, for the Divine Office is a true and real prayer, not a mere vocal exercise. Hence, a priest reading his office as a mere study or as a means of remembering the words of the psalms does not validly recite his office (St. Alph., n. 176). Now, what sort of intention is best and what sort of intention is necessary? An actual, explicit intention which states expressly when the Breviary is opened, "I intend to pray," is the best intention. The devout recital of the prayer "_Aperi Domine_" expresses well the best form of the actual, explicit intentions of those reciting the office. But such an express, actual intention is not necessary; a virtual intention, which finds expression in the opening of the Breviary to recite the office, suffices. The mere opening of the book, the finding out of the office, the arrangement of the book markers, are ample evidence of the existence of a virtual intention quite sufficient for the valid recitation of the office. St. Alphonsus writes, "_Imo puto semper adesse exercite, intentionem actualem implendi officium_" (n. 176). This question of intention gives great trouble to the timid and scrupulous, whose doubts and difficulties seem hard to solve. The common sense and common practice in everyday affairs seem to desert some people when they prepare to read the canonical hours. For, who has not seen the nervous, pious, anxious cleric, stupidly labouring to acquire even a sufficient intention before beginning his hours? Attention in reading the hours is a much more discussed and much more difficult mental effort. It means the application of the mind to the thing in which we are engaged. When we listen to a conversation or when we write a letter the mind is fixed and attentive to the matter spoken or written. Intention is an act of the will; attention is an act of the understanding. 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.