Objection 1. It would seem that attention is a necessary condition of prayer. It is written (John 4:24): "God is a spirit, and they that adore Him must adore Him in spirit and in truth." But prayer is not in spirit unless it be attentive. Therefore attention is a necessary condition of prayer.
Objection 2. Further, prayer is "the ascent of the mind to God" [Damascene, De Fide Orth. iii, 24. But the mind does not ascend to God if the prayer is inattentive. Therefore attention is a necessary condition of prayer.
Objection 3. Further, it is a necessary condition of prayer that it should be altogether sinless. Now if a man allows his mind to wander while praying he is not free of sin, for he seems to make light of God; even as if he were to speak to another man without attending to what he was saying. Hence Basil says [De Constit. Monach. i] that the "Divine assistance is to be implored, not lightly, nor with a mind wandering hither and thither: because he that prays thus not only will not obtain what he asks, nay rather will he provoke God to anger." Therefore it would seem a necessary condition of prayer that it should be attentive.
On the contrary, Even holy men sometimes suffer from a wandering of the mind when they pray, according to Psalm 39:13, "My heart hath forsaken me."
I answer that, This question applies chiefly to vocal prayer. Accordingly we must observe that a thing is necessary in two ways. First, a thing is necessary because thereby the end is better obtained: and thus attention is absolutely necessary for prayer. Secondly, a thing is said to be necessary when without it something cannot obtain its effect. Now the effect of prayer is threefold. The first is an effect which is common to all acts quickened by charity, and this is merit. On order to realize this effect, it is not necessary that prayer should be attentive throughout; because the force of the original intention with which one sets about praying renders the whole prayer meritorious, as is the case with other meritorious acts. The second effect of prayer is proper thereto, and consists in impetration: and again the original intention, to which God looks chiefly, suffices to obtain this effect. But if the original intention is lacking, prayer lacks both merit and impetration: because, as Gregory [Hugh St. Victor, Expos. in Reg. S. Aug. iii] says, "God hears not the prayer of those who pay no attention to their prayer." The third effect of prayer is that which it produces at once; this is the spiritual refreshment of the mind, and for this effect attention is a necessary condition: wherefore it is written (1 Corinthians 14:14): "If I pray in a tongue... my understanding is without fruit."
It must be observed, however, that there are three kinds of attention that can be brought to vocal prayer: one which attends to the words, lest we say them wrong, another which attends to the sense of the words, and a third, which attends to the end of prayer, namely, God, and to the thing we are praying for. That last kind of attention is most necessary, and even idiots are capable of it. Moreover this attention, whereby the mind is fixed on God, is sometimes so strong that the mind forgets all other things, as Hugh of St. Victor states [De Modo Orandi ii].
Reply to Objection 1. To pray in spirit and in truth is to set about praying through the instigation of the Spirit, even though afterwards the mind wander through weakness.
Reply to Objection 2. The human mind is unable to remain aloft for long on account of the weakness of nature, because human weakness weighs down the soul to the level of inferior things: and hence it is that when, while praying, the mind ascends to God by contemplation, of a sudden it wanders off through weakness.
Reply to Objection 3. Purposely to allow one's mind to wander in prayer is sinful and hinders the prayer from having fruit. It is against this that Augustine says in his Rule (Ep. ccxi): "When you pray God with psalms and hymns, let your mind attend to that which your lips pronounce." But to wander in mind unintentionally does not deprive prayer of its fruit. Hence Basil says (De Constit. Monach. i): "If you are so truly weakened by sin that you are unable to pray attentively, strive as much as you can to curb yourself, and God will pardon you, seeing that you are unable to stand in His presence in a becoming manner, not through negligence but through frailty."
Source: Summa Theologica (New Advent)