[Question:]{.underline} Can one retroactively offer for an additional intention a rosary that has already been completed?
[Answer:]{.underline} It is certainly true that for all of our prayers, as with the Masses at which we assist, we can have multiple intentions, and that each additional intention does not take away from the others. However, common sense indicates that the intention must precede the act, for if it did not do so, it could not give the purpose or morality to the act, nor could it be the final cause for which the act is performed. If we perform an act of kindness, it is the end for which it is performed (finis operantis) which is the principal circumstances that determines its value and its merit. The same can be said of our prayers and rosaries. The intentions are the reason why we perform such acts and consequently, if they are to have any influence on our prayers, the intentions must be formulated, preferably at the beginning, but at the very least before the end of those prayers or rosaries. Otherwise they can in no way be considered to be the intentions of such prayers.
This principle is confirmed by the teaching of the moral theologians concerning the intentions of a priest who is offering up the Holy Sacrifice of the Mass. This intention for which the Mass is to be applied must be formulated before the Mass. It does not have to be formulated immediately beforehand, but can be formulated long in advance. However, at the very latest it must be formulated by the time the essence of the sacrifice of the Mass is accomplished, that is during the canon of the Mass, before the second consecration. Otherwise, it can in no wise be considered as the intention for which the Mass is applied. (Prummer, Man. Th. Mor. III. p. 183).
In like manner, the faithful ought to formulate the intentions for their rosaries, prayers and Masses ahead of time, or at least as they begin these exercises. It is good to have a general intention, in addition to the particular intentions, as for example, for such and such a sick person. Such a general intention could be for all those who recommend themselves to our prayers, or for the Church and for souls in need. Another such general intention that can be recommended is that given by St. Louis Grignon de Montfort in the practice of the True Devotion, namely doing all our actions and prayers for Mary as for our proximate end, that is for her intentions, since we give to her the value of all our good actions, and leave to her the entire and full right of disposing of them. If such be our general intention, we need no longer be concerned about having missed out on a particular intention, either because we forgot to apply it, or because we did not know about it. Since Mary, glorious in heaven, knows all such intentions, she can apply our prayers for these intentions, and we would never have a retroactive intention to apply.
Answered by Father Peter Scott, SSPX.